I had a similar problem years ago with my B4. I found that the problem was the transmission case.

I was bolting the two halves too tight, as such, causing the transmission case to warp when bolted closed... It was causing one side to bind, as others have suggested...

If your bearings seem fine, try loosening up the transmission bolts (not too loose that it's coming apart). Try gunning it and see if it tracks straight...

If the transmission case is too tightly bolted, loosen up the bolt but apply a small amount of thread lock at the end of the bolts to make sure they don't back out of the motor mount...