bad vibration not felt in steering wheel

[DRAGIN]

after i got my 94 t/a back from having the rear intake gasket replaced i drove it home (30 miles) and it sat in the shop for 2 weeks, i did not drive it in that time. coming home i did not notice any vibration.next time out i noticed a nasty vibration felt ANY time i drive it on any type pavement etc. only between the speeds of 45 to 60 mph. i feel this vibration in the body but have zero feel of it in the steering wheel so i assumed it was rear wheel balance problem i took it to my local "trusted" tire dealer and they said the balance was off on both rear wheels so i asked them to rebalance and to duct tape the stick on weights on the wheel.i left there and vibration was identical to before, i went back and was assured they did it properly and that their balance machine would haven picked up any flat spot issues in the rear wheels also.i left there and went 35 miles to discount tire.told them i had a bad vibration in the car that was only felt between 45 and 60 mph.i did NOT tell them i had just had the rears balanced just to see what they found.discount assured me it was my rear wheels out of balance and they re balanced them.i told the manager about just having the new balance elsewhere and his tech assured me the balance was off on both tires a good bit.i left hoping it was fixed, and as soon as i hit 45 on the highway it was there just as always.i do not have access to another set of wheels and tires to try and see if putting them on would fix it. the vibration is real bad and it is NOT present until exactly 45 mph on the speedometer after around 60 mph it completely goes away.tires are BFG radials,wheels are 1 piece cast aluminum.any help will be greatly appreciated. thanks

[98 'Bird]

Yeah, I learned to never trust a discount service center. I think most would agree it's best to find a trusted mechanic to do work you can't do yourself. I just don't think anyone should have to deal with those people, especially when it comes to a prized TA. And I've had a bad experience myself .

So you feel the vibration kind of from under your seat? Have you checked your u-joints? I'm no mechanic but that would definitely be a good place to start I think if you haven't already because they're easy to check and find out right away. If they feel good, that's one simple possibility ruled out.

[FTA1995]

This might be a push but what is yur final drive? If you have the stock drive with the stock driveshaft this most likely isnt your problem since you probably have the 2.90 (i dont remember the exact ratios for the autos) but if you have a bigger gear in the rear with the stock DS you could have pitched a weight off of it. Do the diagnosis again using what you told us; logically speaking 19/20 times a speed specific vibration comes from a driveline component output shaft back, UNLESS you somehow damaged one of the drives for a specific gear. Does it vibrate at 45 but then stop at 60 for example? If not, it could be anything output shaft back. I also have another idea; is there ANY possibility that ANY foreign substance physically got into the rear tires? Like fix-a-flat or any other type of liquid? If it sat for an extended period of time with that stuff in there, and if there is nough, it will definitely give you those exact symptoms. Find a way to test your driveline independently from everything else, maybe throw it on a lift or a few jack stands and get that speedo up to 45. if you get that vibe still, its gotta be trans to wheels somewhere.

[DRAGIN]

still working on vibration...

today i decided to get the rear tires off the ground and work on possibilities. i have a vibration in the same spot but it is not as rough as on the road. with the car at 50 the driver side wheel and tire looked to be spinnng true.but the passenger side appeared to me to be a little wavy.but i removed both left and right wheels and tires from the car put the lug nuts back on the studs all full tight and cranked the car again and brought the speed up and it still had a little vibration at 50, (equal to with the tires on) still not rough like on the road.i noticed the wheel that seemed to be slight wavy has a lot of weights for balance on it. they are in 3 different places on the rear of the wheel . i am going in the morn. to see another tire dealer, they deal more in performance wheels and tires and have the wheel and tire seperated and the wheel checked for true and why so many weights. and get a look at the tire off the rim. if that aint it i will move on. still like your ideas if you got 'em
thanks