Can you believe it, this is the 3rd driveshaft for my 82. It just drinks driveshafts, whatever is going on under my car just tears up the u-joints. Thing started to have 'that' vibration through the drive-line and sure 'nuff the rear u-joint was, well i'll just say there was nothing left of the needle bearings, zero.

 

I went through several this winter. Buy a good used OEM one, within a handful of miles it's garbage.

Check your driveline angles. The relative angle between the pinion and driveshaft, and driveshaft and transmission, should be the same. One can be up and one down or both up but they need to be the same within a degree or so. My car had the front U-joint with maybe 2 degrees of angle and the rear U-joint with about 10 Insta-fry the U-joints.

Oddly enough it's bad to have no U-joint angle at all, the needles have to move or they will brinnell the yokes and things go downhill quickly as well.

 

A lowered car or a worn tranny mount can cause this misalignment.

 

Yea Mark, I'd start looking at the driveline. Engine and trans mounts, make sure theres no gross misalignment from any previous "incidents". IIRC your car wasnt severly lowered so that shouldnt be an issue for you.

 

A sagged trans mount wouldn't accomplish more than a half-degree, maybe a degree's worth.

Rear axle bushings gone non-concentric (permanent set) could do it. So could the upper link mounts pulling away from the body. (In my case, some dumbass put in some custom bits and made a simple mathematical mistake that made the upper an inch too short, and further never bothered to verify driveline angles... whatta dumbass )

edit: My car has a loose rear U-joint, but it only shakes in 5th gear. 1st through 4th are no problem. Bearings are noisy, too. Oddly enough, I put this trans in a few years ago because the last one had noisy bearings, and it shook pretty bad in 5th gear, mainly at the speed ranges where you notice a U-joint going out... Cause or effect?

 

I'm not sure of the alignment angels, in degree's . I'm sure the transmission mount is in good shape and nothing has moved in the rear. I do understand that the rearend over time may slip or twist. I do spray WD-40 on the rear bushings from time to time to keep them soft. The rear sway-bar bushings are pretty shot. The "play" in the 3rd member is okay also. i'm kinda thinking it's the size of my tires, there pretty big T/A's
205/60/15's wouldn't the u-joint(s) be the weakiest link in the driveline ?

For the people new to rx7's,,, if you have a vibration "through" the car, first check the u-joint(s) on the driveshaft.

 

It's rather unlikely for it to twist.

Measuring angles is easy, if you have a protractor, just drop the driveshaft and measure the angle against the pinion flange, and against the end of the trans output.