I know this is an old question, but since I recently had the same problem I wanted to share the solution I found with everyone. I was also having problems with my rear left wheel, the gear kept getting stripped out after about 30 minutes of use. I went through 3-4 wheels before I found the true problem. The wheel is mounted to the body/frame of the mower, which is just sheet metal. From mowing sideways across the face of hillsides the mower's weight all goes onto the lower back wheel, and after a year or two of use the body/frame has bent. Now the wheel is not square with the drive shaft/pinion. This causes the gears to not mesh properly and the wheel gear to strip out. I bent the frame back then welded some 1/4" plate to the inside of the body/frame to stiffen it up. Since then the self propel system is working great.

It's not hard at all to repair. I had to change mine last week after 7 years of use. Luckily I had a spare that I bought 2-3 years ago so no trip to the dealer. I would suggest that you get the service manual from Toro's website and save it to your computer and while you are there get the operators manual also. They come in handy. If you are mechanically minded, the repair from start to washing your hands when done, should take you no more than an hour and that is going at a slow pace. When you do go and get a new belt, I would suggest that you get 2 traction belts and another auger belt. After 5 years of use, a belt is starting to get worn out. So while you have the machine apart replace both belts and then 5 years down the road plan on getting some new belts again and replace them. Take care

Though it is the same shaft, this wheel clutch can have several issues as to why it isn't pulling its weight. Best to print a pic from Toro parts, disassemble and have a look.

Solved my own problem.... Found a small piece of hardwood mulch wedged between the drive belt and pulley wheel preventing the belt from doing its job. Removed that and everything works great!