My dryer works on med heat but not high heat, what could be wrong?

I would think that the dryer is too dirty to run on high heat - it would overheat. You will have to tear it down and clean every air duct. Anything you miss will keep it from working. If you have thermistors you will have to test them, too. Here are some manuals: https://dl.dropbox.com/u/8010163/Kenmore%20Dryer%20Manuals.zip
If you tear it down you should have a new belt and some white lithium grease to grease all bearings and support rollers.