i have a vibration at 40 mph and at 65mph. the 33s a/t are balanced and it still vibrates with the 29's. today i got under the truck to look for a worn u joint.theres up and down play in the centre when i push up on the driveshafts. but theres no side to side play.

 

Most two piece shafts have slight up and down play in the rubber. If it's falling out thats nfg. Pull the shaft and feel the joints maybe one is frozen.