so i've been having a shaking in the rear, felt like my tires needed to be balanced, so i checked them out, and each tire is missing quite a bit of rubber, making them out of round, so i decided to get new tires. took away some of the shaking, but the majority is still there, i was wondering what i should do to fix this. it shakes just slightly when i coast at high speeds, but shakes pretty bad when i put on more gas. so i guess its not tires, its not struts cuz i just changed the cartridges, what is it now? i'm all open. thanks.

Have you checked all uni joints?

You should also check tailshaft balance - it could have thrown a weight.

You might also want to check the diff mounts out carefully while you are under the car and the suspension bushings as well. It could be the mustache bar bushings and front diff mount are shot and allowing the diff to move quite a bit which is transferring into the suspension and chassis.

It could be a u-joint and with bad bushings in the mustache bar it is amplifying the problem, by transfering the shake to the diff....

gentlemen, it looks like we may have a winner. after reading your posts, i started reading in my haynes book, and lo and behold, under propeller shaft-universal joints, it says, "wear is characterized by a 'clonk', metallic squeaking and grating/shrieking sounds." which, if you all remember any of my old posts, my car has plenty. so i crawled under the car, gripped the propeller shaft before and after the foreward u-joint and gave it a twist, it has a whole lot of play, the rear one has none. i have very little money to buy a new propeller shaft assembly. so now my question is: does anyone have a picture WITH labels of the whole u-joint? cuz the book just starts spewing out things like race, spider, journals. and i have no idea what they're talking about. thanks!

The drive shaft should be fine (as long as there are no big dents in it) and most likely doesn't need to be replaced. You can replace the u-joints, though. They shouldn't set you back more than $15-20 each. A vice, hammer, and a little patience should be all you need to replace them yourself.

well things went great (mostly) got the new joints, took out the old ones, put in the new ones, and viola, it no longer does what i was talking about, so i'm mighty happy, although it still doesn't run along as smoothly as any other car i've had:( is this just the way a Z goes? with new struts, new tires, and now the prob with the u-joint fixed, i had imagined a much smoother ride. thank you all very much for your help.

Did you check the u-joints on the half shafts ? You really need to look closely , I had 2 bad ones and only saw one , so I went back under a few days an d found the secound one. I then just replaced them all. If you find that the u-joints are good check the rear axel berrings while you are up in the air.

well, i checked the rear u joints, and there isn't anything similar to the front propeller shaft u-joint, the rear ones are all tight. but it feels like theres a prob in the differential, its got a looseness, a clunk, like i had gotten from the u-joint, but its inside the differential. is this normal, what can i do?

Replace the diff. or at least rebuild it.