Vibration at 70 mph????

[nfran2]

I have a question regarding my 1978 Camaro Z28 & the rear end. I have a 3.73 Eaton Posi original rear end in my car. The fluid has been changed regularly every 5-6000 miles since I bought the car new. The car only has 58K. It now has a ZZ430 clone 350 and original Borg Warner T-10 4 speed. The axles were changed with Moser C-clips about 5 years ago but the rest of the rear end is all original. The original axles were out of round (twisted) necessitating their replacement. This is when the vibration issue started, 2004.
When traveling about 70 mph, the car begins to vibrate excessively. When putting the car in neutral at 70+, the vibes are still there. The engine has been professional rebuilt and balanced as well as the flywheel & pressure plate balanced. The drive shaft was balanced also. The car has 4 brand new BFG tires which balanced out fine on the original Z28 steel wheels.
Could there be anything going on in the rear end that would cause this type of vibration? Anything below 65 mph and the car runs great. I live in Fl where most of the interstates and expressways are 70mph. The car has never been raced but driven aggressively.
Any help or suggestions as to what to look for would be great appreciated.

[Me-109]

From your description, it sure sounds like some kind of issue with the rear axle and the axle shafts. How did the original axle shafts get twisted?

Originally Posted by nfran2

I have a question regarding my 1978 Camaro Z28 & the rear end. I have a 3.73 Eaton Posi original rear end in my car. The fluid has been changed regularly every 5-6000 miles since I bought the car new. The car only has 58K. It now has a ZZ430 clone 350 and original Borg Warner T-10 4 speed. The axles were changed with Moser C-clips about 5 years ago but the rest of the rear end is all original. The original axles were out of round (twisted) necessitating their replacement. This is when the vibration issue started, 2004.
When traveling about 70 mph, the car begins to vibrate excessively. When putting the car in neutral at 70+, the vibes are still there. The engine has been professional rebuilt and balanced as well as the flywheel & pressure plate balanced. The drive shaft was balanced also. The car has 4 brand new BFG tires which balanced out fine on the original Z28 steel wheels.
Could there be anything going on in the rear end that would cause this type of vibration? Anything below 65 mph and the car runs great. I live in Fl where most of the interstates and expressways are 70mph. The car has never been raced but driven aggressively.
Any help or suggestions as to what to look for would be great appreciated.

[Forgiven]

If all else fails revisit the drive shaft, mabe a weight fell off or it may not have been done correctly.

[Mase]

Sounds like a tire is out of balance. start there.

[nfran2]

Me-109, I bought my Z brand new in Oct 1977, & after driving the car for 20+ years aggressive and hard on the street, can't tell you why the axles were twisted and never snapped. I remember taking the originals out and putting them on a center-to-center lathe, the round out was almost .75"! At the time we did this, the rest of the axle looked good, bearings, seals, posi, etc.
Forgiven, you're reading my mind, definitely going to have the shaft rebalanced, done about 6 years ago but, ya never know.
Mase..... tires are brand new and balanced so I'm going rule them out for now!
Just trying to get an idean where to start before tearing the rear end apart!

[nemocamaro]

alignment, wheel bearings,,,

[Mase]

Bad tire and I still wouldnt rule out balance even on new tires. Just me I've seen brand new tires come back unbalance before.

[79-85i-95z]

check the tailshaft bushing in the trans. always over looked.

[Greg Norris]

how old is the suspension on it? a lot of cars get a vibration at speeds over 70.