What's the model of the mower? I'd start with looking at the teeth on the wheel and the pinion gear on the driveshaft that drives the wheels. Depending on your model, there may be a spring and woodruff key in the driveshaft under the pinion (make sure not to loose these).

Remove the belt cover and see if the belt is engaging. If the belt looks worn, replace it. Check the driveshaft output bearings on the inside wall of the deck. If they appear to be in bad shape, replace them as well.

I recently replaced the entire transmission assembly and drive shaft output bearings on my Toro recycler. Was pretty easy and you need snap ring pliers. Runs like new now.

Thanks for your response Woody71!
The transmission is working fine....I took off the wheels and when engaged, the shaft works fine. The problem is the plastic wheels and possibly the gears at the ends. The plastic 'gear' in each wheel is separated from the rest of the wheel.....do you know if they are interchangeable with the front..they seem to have the same construction? If not, where can I get them cheap? Also, the gears at the end of the shaft look worn; the left side worn in the middle, and the right side worn on the ends.

You can find the wheels at some of the larger Home Depot stores, from a dealer or online. I would replace both pinion gears as well. The wheels aren't interchangeable as the front don't have the teeth. When you replace the pinion gears, clean everything and apply small coating of Anti-Seize.

Thanks again Woody71....I will take your advice.
I appreciate your help!

This message is for Woody71.......
Woody, at what points should I put the anti-seize?
BTW...the wheels ARE interchangeable! That could save someone some money.......all you need to do is take the back wheels, pop the rivets out of the plastic and position the metal gear onto the front wheels and hammer the rivets into the designated holes! The only problem is the i.d. of the back wheels is a bit larger to accommodate the metal bushings, but all you need to do is hone a bit larger.