vibration from the rear

[xero1]

i got a vibration from the rear at 55 mph and up when i accelerate. i have a 95 240 without abs with manual transmission that i just put in. the drive shaft is a one piece drive shaft shop drive shaft(was rebalanced), 350z front and rear rims and stock 350z tires(tires are balance and round), Eibach Sportline springs, kyb rear struts, stock front struts, 300zx front and rear brake upgrade, 5 lug conversion. i took the rear axles apart to look for wear, and the was no major wear in the half shafts. i swapped the half shafts and change the grease. I still have the same vibration. The car have 134k miles, and it had bent rims when i got it. Anyone had this problem?

[skydragoness]

When does it occur? I have vibration in my car, its not coming from the rear but it resonates and vibrates the whole chassis. It only happens in 5th gear when I am not on the throttle, if I apply throttle then it goes away. Nistech figures the 5th gear on my transmission has a bad countershaft bearing. Its a known technical service bulletin for the S13. Don't know about S14's though.

[xero1]

i drove the car home yesterday, the vibration is at 55 mph in 4 or 5th gear from the rear, but it get better when i'm 60mph and up, also the car dosen't vibrate under 50 mph, at 80 mph there is no vibration(4 or 5th gear). i'm thinking the tires are bad. front tires are cuped(i fliped the tire front tires to try to fix this) rear tires are round and are not cuped. all tires are round and are balance. do the nico members think the tires are causing the vibration?