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.

The one way wheel problem is not a problem, as that's the way it works. Since it is self-propelled, you wouldn't normally try to pull it back, unless you bear down on the handle to lift the powered wheels off the ground. Most lawn mowers sold to home owners are not designed for much more than a weekly mow. And, since your son does lots of mows every week, the life span of the mower has been shortened considerably! The transmission parts, or a new tranny, are quite expensive. You must consider if it is worth repairing, or just buying a new one! Many youthful lawn mowing businesses have been shut down by the kid using the family mower, and wearing it out much before its time! Now is the time for the son to learn the facts of life! To wit: Using the family lawn mower for contract or friendly mowing jobs, will not gain favors from Pops! So, that said, have the son buy a heavy duty lawn mower that he can use (or buy the first one for him) for his business. He must save part of the cash he gets, to buy stuff he needs, and that will teach him something more than what he learns in school! And also, he must learn that he needs to save money to buy the new one, when the old one wears out. I learned long ago, that money talks. And, spending it on the latest fads usually begets you nothing more than being broke most of the time! And/or, on welfare!

You may have better luck rolling the wheel adjusters all the way down to lessen tension allowing the axle to shift. If it is stubborn, a wood worker's pipe clamp will move it over.

When I do it myself I use BananaBoat. When I have it professionally done I go to Mystic Tan. It is the booth thing and it is wonderful. It takes less than 5 minutes for your entire body and does not streak or smear and goes on evenly. You can also buy the airbrush equipment (from an art store) and order the liquid tan stuff on-line and have someone spray it on you (I've also done this). It takes a little practice to get it even, but not much. You do need someone else to do it for you, though, because you can't reach your back and the back of your legs easily. Also, you need a space that you don't mind getting some of the excess tan stuff on. We did it in our garage. I can't remember where I oredered my tanning stuff from, but I found it by doing a search for airbrush tanning supplies.