It could be any number of things. Mine used to feel bad under acceleration and it turned out to be bad tires/ improper tire inflation. If ti is really bad, look at the bushings. It's over 20 years old and rubber parts are usually the first things to go. Also, live axle cars experience "axle jacking" under hard cornering loads, and the rear can jump out when it hits a bump because of uneven contact patches. It usually does not happen unless you drive like me, but then I do not know how you drive or under which specific conditions this is happening.Just remember that cars are normally more of a handful on gravel than pavement. Inputs are more of a suggestion than a command, and you need to compensate for this. For me, this is fun. I used to try to rally my Cutlass and my Sentra driving 70 mph on dirt and getting them to slide, but I'm nuts. As for the U joint problem, to test it try the following: jack the rear end up and support the axle tubes on jackstands. Next, go under the car and grab the driveshaft. Now try to turn it. check to see that there is no play between the movement of the driveshaft and the yokes. There will be some movement in the rear axle as the pinion takes up the freeplay with the ring gear, so be sure that you are looking at the yoke to driveshaft relationship and not just going for gross movement. If it "clunks" into gear when you are driving it, it is likely the U joints. That's what mine did when the joints were failing.
