Still having vibration issues...please help!

Hey folks, I posted once before about my car vibrating while going up hill. I have an '86 5.0, 5 speed manual. Only does it when you go up hill, and on the interstate when it does it, it feels like the drive shaft is going to fall out. Sounds like something thumping under the tranny too. When I let off the gas, it instantly stops. The other thing is it doesn't do it all the time, but when it does, wow is it bad.

I've checked the U-joints, and I checked the bolts at the rear end flange (at the drive shaft) and they are tight. Tranny mount is ok. Also, there is just a very very slight amount of play up and down where the drive shaft goes into the transmission, but nothing abnormal. If it were that, the seal would be beat all to pieces, and it is fine. What could be causing this? It's driving me insane and I need to get it fixed. It could be at the rear end for all I know and just sound like it's at the transmission. I'm not ruling anything out at this point.

Thanks in advance for your advice!

gerzen said:

it could be the drive shaft is bended or the tail shaft of the trany is bend it, i have that problem ones in my stang the vibration go up at 50 mph and desapears at 65 if you have the vibration only at that range of speed 50 apears and up to 65 dessapears, you have a balance vibration it could be be the drive shaft the tires, etc.

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Well, it only does it going up hill. No particular speed...sometimes it does it at 40, sometimes not. On the interstate it only does it going up hill. Down hill, even when I am pressing the gas pedal it doesn't do it. Going up hill when it is vibrating, it stops when I take my foot off the gas. That's why I don't think it's tires.

I also checked the drive shaft and all the weights are on there.