Bad vibe
 

Between 50 and 70 mph, I get a pretty bad vibration in the car through the stick. Below 50mph and above 70 mph, it drives like a dream. Also, when I hit about 85mph, I get a nasty rattle that I can feel in the stick and hear. It sounds like the tranny is about to fall apart. When I decelerate, it stops rattling at about 70mph.
It doesn't matter if I put it into neutral to coast down, or if I ride the gears down. Any ideas or corrective suggestions would be greatly apreciated. Every time I hit about 85mph, it sounds like the something is about fo fall out. Thanks in advance for the help.
Caymon

Happened to my son's 89 the exact same way. His had about 180,000 miles on it though. When it gave up the ghost, the 5th and reverse gears took a s*** and the main shaft pretty much destroyed itself in the process. Check the tail shaft of the transmission, THere's a sleeve in there that the front yoke slides into that wears out or often slides out and lets the yoke bang around in there until it eats the transmission. When you do this you might look at the yoke and see if there's a worn spot on one side and check the U-joints as well. I'm not sure what caused ours to go south because it pretty much destroyed everything in the process. One ominus sign is damage to the rear transmission seal.

I've got about 185,000 miles on the car, but the tranny is only about 5000 miles. The last time it went, I lost 3rd and 4th. The guy told me that the ain shaft broke. If it did, wouldn't that prevent you from going anywhere? Back to the point, I have had the tailshaft bushing and housing go to sh** on me before. I hope it is not the tranny again. If so, does anyone know where I can get a good deal on a tremec? I have gone through 3 t-5's in the 2 years that I have had this car. Thanks for the response.
Caymon