I have a pretty newish consumer type Toro 21" self propelled mower with the personal pace type setup that has had one of the wheels' drive system go out. I've taken it apart and even replaced the moving key and gear in the wheel assembly with ones from an old unit that worked and it still slips. The other wheel drives fine. What am I not seeing that is broken? Could it be inside the gearbox itself? I do recall it not wanting to go into neutral and reverse sometimes right after I got it, and I wound up dragging it backwards a few times. I prefer the squeeze type drive system found on the commercial models and older recycler II's but this unit was bought for a 2nd home that I no longer have and I wanted to get some use out of it. Runs and cuts great, I just prefer having two wheels working. Two previous consumer models are 10 years+ and had zero drive problems. This one was broken within a few months.