94mustang94

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?

AdderMk2

balance your tires

nba1341

check wheel bearings?

Jfsram

Check tires/rims for runout.