i have a 68 with a fresh 385 fastburn to a built th350 with a 2300 covertr im gettin a vibration underneath the u joints are new i did notice some play in the front of the driveshaft near the outshaft on the tranny should i just buy a new yoke and driveshaft or is it worth it to balance the original also if anyone else has any ideas im open for suggestions thanks alot

Are you running a Poly trans mount? If so I have seen this cause problems as it raises the rear of the trans just enough to change the relationship angles between the two U-Joints. This doesn't always cause a problem as mine is fine but Chris (CFunk) had this problem.

mounts are new but either way there shouldnt be "play" between the drivshaft and tail shaft right??

There is a brass bushing in the tail shaft of the transmission that the yoke rides in. You should probably replace it. Sounds like it is worn. Your drive shaft vibration might be coming from a distorted shaft at speed. These shafts were made for lower RPMs and when lower rear end gears are installed the RPMs of the shaft are more than it was designed for. The shaft might look fine when it is still, but at higher RPMs it will bow and go out of ballance. I see this a lot with people who have installed lower rear end gears and an over drive transmission.

gears are 373 posi spinning 17" wheels hotchkiss lowering suspension