so anyways, here's the important info. just bought it. 91 gxl, 110k on chassis, new poly engine mounts.

it feels like there's slack in the drivetrain, it's quite noticeable at low speeds especially when starting from a stop. it feels like if you launch just the right way, it's not so noticable, like when you ride the clutch a little. the car jerks back and forth similar to how a fwd car with bad motor mounts would. the higher the speed, the less noticeable it is.

so far i've figured out i need to check the diff mounts, rear diff freeplay, and u-joints. now what i'm also confused on is i've heard that the u-joints are not replaceable and you have to get a whole new driveshaft, but i've also heard that if you have the right tools, you actually can replace the u-joints. what's the deal?

i'm currently going to a large local auto tech school with what seems like every tool on earth at my disposal. (for example, i was just using a $40,000 valve seat and guide cutter today)

 

The first- and easiest- step would be to replace all the suspension bushes...a-arms, tranny mounts, subframe bushes, etc.
During this process you'll have the driveshaft off and can inspect the u-joints.
Mazda did not intend the u-joints to be replaceable but I've read that it can be done, so search the forum and you'll find out how.