5 spd D/R transmission vibration

I have an '88 five speed D/R in my '84 Brat, along with it came a vibration from about 30 mph and just gets worse as speed increases. I didn't have the vibration with the four speed, and it's not there without the driveshaft, so that eliminates the rear axle as the source. The new one piece driveshaft was balanced when it was fabricated, and I had the balance rechecked on Wednesday, the driveshaft shop assures me that's not the problem. It's still there under power or coasting, with the clutch in or out. So I'm assuming the vibration is from the transmission, probably a bad "mainshaft" bearing. I lost one ("mainshaft bearing" the dealer called it) in my '05 Legacy (five speed) and they replaced it (under warranty) without removing the transaxle, can I do the same with this older transmission? Has anyone ever done it?