When I leave the shifter in third to prevent the upshift to overdrive, I get some very serious vibration when I let off of the gas over 60 mph or so. Does this happen to anyone else? I do not get the same problem if I let it shift to overdrive and decelerate from the same RPM (higher mph).

 

Possible either your tranny mount is shot or maybe your driveshaft is out of balance. Both are known to cause vibration. Hopefully it isn't your 3rd gear going out or anything, but if your problem free besides that I doubt it is. Someone else can probably answer better. Check that tranny mount though, it's kinda a notorious problem with these cars.

 

I have been getting the same vibration. I thought it was my tires (same were dry rotting as the car was sitting for awhile) and I just put new BFG's on and have the same problem.
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I don't notice anything until I hit 60-70 mph in overdrive I get a bad vibration and it is worse when I let off of the gas. I don't get it in lower speeds. It just had the U-Joints replaced about 4-5 months ago and this is the first time that I have been traveling at that speed. The tranny seems to shift fine though.

Does anybody have a clue or know a good place to check out a driveshaft in Upper Bucks County PA???????????

 

When I had my rear redone last year ( labor under warranty -- I bought the Eaton and Precision 3.73's ) they said the u-joints were bad. Seriously bad vibration above 65mph after that. Seems the shop bent something when replacing the u-joints. So I got an AL shaft and bent that when the tranny mount broke 3 months later.....