U Joints.
 

Are you joints hard to replace?

I think you have to replace the driveshaft. You can buy an aftermarket driveshaft that has replaceable joints.

http://rx7world.com.phtemp.com/catal...2_products.htm

he is right. stock driveshafts have non replaceble u joints

It's not hard, but it takes a little time to do it. This write-up might be helpful...

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/replacing-non-replaceable-u-joints-517657/

mazdatrix.com has a nice one with replacable u-joints

I'd just get a new driveshaft.. specifically the aluminum one.... mmm... sexy...