Help! Drivetrain noise/vibration over 40mph.

[justgio928]

I have an 88 DLX that gets a weird drivetrain noise/vibration whenever I get over 40 mph. The reason I think it isn't just the tires (265/70/16 Guard Dogs) is that if I let off the gas, the extra noise and vibration goes away even when I'm still over 40 mph.
Any ideas on what would be causing this? Do the gears in the transmission stop turning or something when you let off the gas even without pushing in the clutch?
I recently checked and topped off trans oils for the trans and rear diff and noticed that it was dino sludge in the rear diff. Should I flush it all?

Thanks,
Justin

 

[KidVermicious]

Um, yeah, sludge is bad. Change it. Unfortunately, the damage might already be done.

And just to rule out everything else, get your tires rotated and balanced and have your rear driveshaft balanced.

 

[DC runner]

Sludge is bad, mkay.

 

[justgio928]

Yeah I forgot to mention that the vibration occurs throughout every gear regardless of the rpm of the engine. Just once I get over 40 mph. The transmission has been iffy on it from when I first bought it but this seems to not be a result of the trans but rather something further down the line.
How do you get your driveshaft balanced? Will most places do it?
The tires have less then a 2000 miles on them and the noise that concerns me goes away when off the gas so I know its not the tires. But yeah I'll probably rotate them sometime soon.

 

[KidVermicious]

Yeah I forgot to mention that the vibration occurs throughout every gear regardless of the rpm of the engine. Just once I get over 40 mph. The transmission has been iffy on it from when I first bought it but this seems to not be a result of the trans but rather something further down the line.
How do you get your driveshaft balanced? Will most places do it?
The tires have less then a 2000 miles on them and the noise that concerns me goes away when off the gas so I know its not the tires. But yeah I'll probably rotate them sometime soon.

No, a standard garage isn't set up to service a driveshaft. Check your yellow pages and call around.