I have 94 f250 4x4 with a 5.8 and a 5spd ZF(light duty) transmission. The truck has 227,000 miles on it and the synchros on 2nd and 3rd have been slightly grinding and getting progressively worse so I know it needs synchros, but the real problem is it sounds like the mainshaft bearings are worn out because any speed over 30 when the clutch is disengaged or on light decel there is a horrible noise from the transmission and vibration transferred through the shifter.

My question is, have any of you rebuilt one of these transmissions? Is it rebuildable or are the main bearings part of the case and not serviceable? Or is it just more cost effective to replace it with a new/known good unit?  

It is rebuildable it requires many special tools and lots of patient.I would recomend a new or reman unit