Toro model 20016 front wheels locked up?

I have had a lot of problems with the front wheel drive system on my Toro. Today, while mowing, the front wheels would not turn when the drive lever was pulled and the wheels tended to lock up. Is there a trouble shooting guide or repair manual that I can use to fix this myself? I'm mechanically inclined, but I like having a manual to refer to in case of trouble. This mower was purchased in 2002 ser # 220327872

Thanks