With the older cars with limited slip rear ends you had to use certain fluid. It might be the cause of your problem. Has it ever been changed? How many hours, miles since the last change? Manual says to use and change the transaxle fluid. Mobilube HD LS 80W-90 (or equivalent) https://www.manualslib.com/manual/878997/Bush-Hog-Th420.html?page=54#manual