When I get on the highway and get up to speed 70 mph in 5th gear I start to hear and feel a vibration. Funny thing is I really don't notice it vibrating much in the first four gears, but I really start to feel it Vibrate in 5th gear!

I take it out of gear and keep it in Neutral and I still hear and feel the Vibration coming from the rear at high speeds.

What I think could be making the Rear Vibrate:

1. Transmission tail shaft bushing?
2. Off Balanced Drive shaft? Stock JG1. (Doesn't feel like the U-joints)
3. Rear-end Bearings?
4. Off Balanced Wheels?
5. Warped tires? (Tires are 3 years old with lots of tread)


I'm crawling under the car tomorrow to double check everything and to rule out #1-5. I'm also going to change the rear-end fluid tomorrow and rotate the tires front to back. Wheels are 15'' so they're not off-set.
If the tail shaft bushing is bad, do I need a pry bar to check it or can I tell by hand? How can I tell if the drive shaft is off balance and how can I tell if the Rear-end Bearings are bad?
Any input would be helpful so when I'm under the car Tomorrow I can diagnose the vibration problem and fix it!

 

Ok.
I got the car off the ground and I felt the rear tires, the axels have about an 1/4'' of play in them...Clonk clonk clonk pulling in and out. And I also feel about a 1/2'' of play when I rotate the rear tires forward and backward hearing it Clink clink clink on the drive shaft gear. I can feel a good gap doing that while touching the drive shaft.
That's not Normal is it? Feels like too Much play!
I put the car in drive and sure enough I hear the clinging of the axels on both sides, that's the vibration I'm feeling from the rear.
My rear end is a 3.08 posi drum. So do I have to re-shim the pinion and/or the carrier shims or both? That should fix it right??

 

That sounds like U-joints.

If you haven't ever changed them out, they probably need it. If they're still the stock ones you'll need a torch to melt the plastic, a BFH, a drift or junk socket of as near as possible the same OD as the U-joint end caps, and a socket or old axle bearing or something to set the other drive shaft ear on to act as a receiver. Once you get the OE ones out, they're about 100 times easier to replace afterwards, because that damn plastic is taken care of. You just gotta be careful not to bend the drive shaft ears until the plastic is cleared.

While you have the shaft out, see how much slop the pinion has. You can try tightening the nut a little bit at a time, until the preload starts to noticeably increase, which might take out another good bit of slop. You need a long breaker bar, a 1¼" socket, and a "special" tool about 4' long, like this one.



Axles are probably fine. I wouldn't re-shim anything just yet.