Alright, so I'm a bit worried. It started out as just a random tug once or twice while accelerating earlier in the week but generally ran fine but a few days later I started it up to head home and I had to deal with shaking and a weird clunking sound near the rear wheels when I steadily accelerated. It's not too bad going through first and second and does alright in third until I accelerate from 40 to 50 MPH and then I have to deal with really intense shaking/jerking unless I step on it and push the RPM's up to 2000+ at which point it almost goes away. At 50+ MPH it runs just fine. I took it in to get my transmission fluid/filter changed and it didn't help at all. Anyone have any idea whats going on? Also, the previous owner did mention a couple times that the transmission has only 40,000-60,000 miles on it because they swapped it with another one from a corvette (didn't really get specific with me) My car is an automatic.

If its shaking try this, with the car in neutral rev engine while going 25-40mph. IS it clunking still?-try turning the car off in neutral going 25-40 and coast?-making noise? next get under the car, and tug and pull the drive shaft,,is it loose? and next put the rear up on stands and with front tires chocked, put the tranny in neutral and turn rear wheels forward. I suspect maybe a bad ujoint on drive shaft, broken spline axle teeth or ring gear diff, and maybe last the dreaded tranny. since its in the back rear, im going to have to go with broken ujoint or missing ujoint strap. I had a rear differential blow out on me. It was in a truck so all i did was remove the driveshaft put her in 4 wheel drive and drove her home. but third gen cars obviously cant do that. so maybe the previous own did a grip of burn outs the night b4 the sale of the car. Happend to me. I almost bought a 81 trans am with 455. the guy blammed his wife for doing donuts and hitting the curb and busted the rear axle and bending the geometry support brackets in the car. If that had never happend to me, then i wouldnt be on this website. its all about the 3rd gens dude.

Could it also be something un-trans related? Like the weights on the wheels to balance them have come off.. bad wear on the tires causing a bulge on them or a broken belt in the tire. Just some quick idea's that came up as thinking of some issues I've had with vehicles before.

Thanks for the replies. To camaroman, I just had my tires mounted and balanced less than a thousand miles ago and it only does it at specfic speeds + RPMS so I doubt it but I will look into that anyway.

To transamdudeman - the mechanic at the shop I work at said he thinks its something with my engine. He took it for a ride and said he really thinks its not the transmission but he thinks I might have some water in my gas but we'll see after I run it out and refill it. He did find a sparkplug wire that was burnt by my headers so I'll be replacing that.

Also, my check engine light has been coming on and off so I'll get that checked out but do I have to plug the computer diagonostics tool straight into my ECM or is there another socket somewhere?

under the dash theres a diagnosing link. you simply take a paper clip, and jump the A and B terminals with the car off!. then turn the key to on position (not start) and watch how many times the engine light flashes. i should flash once then following that, another two flashes =code 12(normal). it does that three times total, then it will flash other codes. count the flashes. and report in. If this is way too confusing, try searching (trouble codes) and maybe its time to get a cheap manual. it has every code in the book. ive had mine for almost 5 years and its falling apart and turning yellow.