I've got a very bad vibration in the rear end and the parts store said it was the u-joint but none of the local parts stores carry u-joints for an 88 rx-7. here's the problem when i accelerate the vibration gets worse a the speed increases if anyone thinks it's anything other than the u-joint I'm all ears

 

The Mazda driveshaft doesn't have replacable U-joints. You need to replace the entire shaft. Mazdatrix makes a shaft with replacable joints, but you can have a local driveline shop make one as well.