Just saw you have a 48" 24hp.
I know what you mean about the belt drive in "reverse". Not much help.
If are in an open area with the mower running, if you put both controls all the way back to the index bar, what happens?
Does it turn a little at first and then sort of straighten out? This could be just the split second timing difference between when both handles hit the bar.
Or does it go in circles? How big are the circles? Does it go in circles if you do the same thing in the forward direction?