drtbiker,
I went through the same thing with my car (89 LX).
I installed 3:55 gears and out of the blue, I got a weird vibration but only at certain speeds (mostly above 50-mph) and like yours, when I let up on the gas, it got worse.
I was convinced it couldn't be the new gear set so I embarked on a month long mission to find the source of vibration. I went as far as balancing the drive shaft, Flange and Yoke and even installed new U-joints. I became obsessive; I (balance) weighed the bolts that hold the drive shaft to the Flange and even marked my wheels and lug nuts to maintain a baseline! I also used a dial indicator to find the run-out/high-spots on the drive shaft, Flange and Yoke. When I re-installed everything, I countered the high spots by rotating the pieces so the high spots were opposite each other. (I learned this from an NVH guy who works at Fords Research and Development Site)
I'm pretty lucky as my brother works at Fords Research and Development site (in Dearborn, Michigan) so whenever I need a tip, it's no further than a phone call away!
Needless to say, the vibration was not eliminated! It was better but not by much. Now, at this point I suspected the new gear set!
So I installed a new 3:73 gear and the car ran smooth as silk.
It just so happens (Again, Lucky Me) one of my friends brother-in-law works at the Ford Axle plant in Sterling Heights, Michigan. His job is to randomly test gears as they come off the production process. He tested my new (Motorsport) 3:55 gear set and indeed it was bad!
Maybe yours is too.... It happens! Like I said, they randomly test them and if/when one comes up bad, they go back and remove so many that were made before the bad one was found. But some still get past the testing process; it's not perfect. I can attest to that!
Phil
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