I have been chasing a vibration problem for over a month now. i replaced the tailshaft bushing and installed the spacer on the driveshfat that is required for the 3550 in my 94. i replaced the u joints 2 weekends ago and it went away. i also pulled the rear cover and gears have a fine wear pattern with no signs of excessive heat. the vibration went away after doing the u joints but now its back...trans mount is fine and everything is tight. it only does it about 60. what else?

 

balance your tires

 

check wheel bearings?

 

Check tires/rims for runout.