1994 Z28 convertible LT1:At 48 to 53 mph in 3rd (auto), there is a distinct vibration/shake present that seems to be in the front end - but also seems to come from the rear under deceleration. All tires are new and re-balanced three times to as perfect as possible. Suspension parts are all good and tight, bushings good and alignment right on. The trans guy said it is the auto trans; drive line guy says it is the drive shaft. Is there a really reliable - no BS - way to check the balance, and trueness of the shaft? I have a feeling that the first place I go to will tell me that a new shaft is needed, and here we go with the parts change mentality.

The car has never been wrecked or raced, so the frame is not tweaked. If I were still living in Indian Wells, CA, I would know where to go;here in Boulder, CO there are only "yuppie mobile" dealers who are into parts swapping and over servicing everything with additives and conditioners!!! lol
Any advice would be sincerely appreciated.

Thanks for your help.