Ok.. so i am having some installation done and when i showed up at the garage the mechanic saw how my car jumped when i put it into drive/reverse.. he said he really didnt like that it did that and said he would look at the driveshaft.. so he gets it up on the lift and has one of hsi guys move the wheel and shows me how much play is in the driveshaft.. then he points at the "rear housing" (i know almost nothing about cars) so the big ball looking thing (i know i sound like a retard) anyway.. so he points and tells me that it is probably going to go soon and i should look into replacing it and advised me to check jegs or other performance sites.. anyway.. what i was wondering is what exactly am i looking for? one of his other guys said it should come as a kit and cost like 200-300 but again not wanting to look like an idiot i didnt ask to specifically tell me what was going bad.. just wondering if anyone can give me a clue as to what they are referring to and what will work in my 87 Iroc 350... im looking on the computer but i dont know what i am lookin for.. thanks

you can usually buy a complete used rear off ebay, or some other sites for less than 500 bux. Any rear from 82-02 from a camaro or firebird will bolt right in, though the 93-02 ones a a bit wider so the wheels will stick out a little more. I dont know how much the car "jumps" but that could be caused by an the idle speed being too high. I find it hard to believe that the rear has so much extra play in the gears that its causing this problem, or that it would even cause this problem.

I would recommend a second opinion from a different shop if you are that concerned with the the rear.

What is the car, and and at what RPM does it idle?

it is a 1987 Iroc-Z that had a 305 stock but previous owner put in a 350.. car has 160,000 miles on it however the engine only has 25,000 and the tranny was rebuilt at 120,000... the car idles at about 1,500rpm on cold start up.. then it goes down to like 1,000 and then once fully warmed up it idles at about 600-800...

my car jumps a lil bit when i put mine in drive/reverse but its only because my clutches in my 700r4 are going back so it has lag time when going into reverse which makes the car jump like that.

The jump was caused mainly by the tranny engagement. 87-later trannys (if you still have the orig one) had added the AuxillaryValveBody inside of them which acted to cushion the harsh engagement.
Higher idle RPMs and older (non Aux V-B) trannys had an abrupt engagement characheristic. Usually the u-joints are the sacrificial lamb due to that hard engagement.
To determine any play in the rear end itself, you would need to remove the driveshaft and feel for play between the pinion (input where the driveshaft mounts to it) and the ring gear (conected to the wheels esentially).
Do you hear a ping sound when engaged and you have the brakes on?

agreed. you may have too much backlash in the rear. The gears may be worn and may need to be replaced.

it probably is the differential. my car does the same thing, but I know it is the differential because if I accelerate quick and get good traction the car makes a loud clunking and does an endo. So i'm lookin for a posi differential with disk brakes right now.

Sounds to me like the guy meant your axle gears.
The pull-it-yourself salvage yards around here offer complete rear axle assemblies for $75. But you gotta go with tools, and with knowledge. Plus half of what you find will be worse than what you have.
If you're not inclined to address the issue yourself, it could run you nearly a grand. Start by getting more opinions on how bad it really is. Not from shops, but from fellow enthusiasts in your area. Even if they drive Mustangs or something.

No, its not likely that your differential gear backlash is too big. if that was the case then when you drive it enough to get the rear end hot, the clunk would be less noticeable because of the heat expansion in the gears, hence the need for ring gear backlash.

the more likely culprit would be the U joints. on my car theyre bad and i get that clunk and harsh engagement and i just got my transmission rebuilt aswell. i havent gone in to check play on the driveshaft but i suggest next time u go get an oil change to have the tech lube up your u joints, if you havent already, and if changes alittle then thats your problem, and u joints are pretty cheap around 20 to 40 bucks depending on which kind. go for greasable ones if you do end up getting some, the non greasable ones just make me shiver to think im relying on factory grease to last me forever.

also if you wanna check your ring gear backlash, pull the back pumpkin cover off and have the car up on the air, have a friend turn the wheel back and forth. you might here alittle lite clunking when changing the direction of the way the wheel turns. but if your dont, try it faster and faster, also if you havent figured out by now or you dont know, backlash is the space between the gears before they mesh, have your ever played with a set of gears and if you push them really against each other they turn alot quieter? but if you hold them farther apart they kinda do mesh all the way into each other? well you want that, just not alot

I know for sure its the diff because i had the car in the air, and put it in drive and reverse to see if it was the u joints which is what i had originally thought, but saw no movement in them, but the d.s. turned almost two full turns before it stopped with the tires off the ground and the brakes on.

did you check for play in the u joints? or grease them? u joints dont make as much noise in the air seeing as how the wheels dont have that much resistance, but then again another thing comes to mind is the torque arm bushing, which goes on the transmission and sometimes when it goes bad it transfers movement from the diff to the car.

the mechanic sounds like he was talking about a ring and pinoin setup from summit which isnt too bad if you know someone that can install it for you or you can do it yourself, the plus, you can get a better gear i.e. positve traction, lockers, world of possibilities, you can also get a nice launch out of a bad situation with a better gear behind an already good engine.

just the ring and pinion setup wouldnt cover everything.

and it doesnt make sense that the rear end gears would be the problem either, i think you should hold on before goin ahead an buying it. but basically what you would be looking for is a differential rebuild kits, which comes with all the bearings and some shims. also new ring and pinion gears, and new axle bearings since they dont come in the rebuild kit.

the u joints aren't greasable, and I watched to see if they had play in them but it was all in the diff. When I put in the posi rear i will put in new greasable u joints. I've got a lead on a differential with disks and posi from a formula firebird. maybe i should just buy the whole car.

have u opened up the rear end to check the backlash between the gears? take off the cover and just move the wheels to see how much backlash you have. its odd and unusual for backlash to just change like that. when did jumping start? after any major work? all of a sudden? or got progressively worse?

i bought the car last may and it has been like this ever since i got it. I put almost 3k miles on it the way it is, if you don't step on the gas it doesn't clunk. but it doesn't matter as i'm getting a whole new differetial and carrier.

My car would jolt like that usually when put into R.

Turned out to be a busted tranny mount.