Step Motor Improvement KitsDorman Window Lift Motor 742-251 at AutozoneTake your complete old step motor into Autozone and see if it matches this one.

It's a simple swap to change the gear from the old one to the new one and swap the connector plug - you're back in business. There are 3 plastic discs under the cover that sometimes crumble. They're there to transmit power from the worm gear driven gear to the output gear. I'll bet yours are nothing but a bunch of course granules now.
See also:
> KWIKEE STEP MOTOR REPLACEMENT SERIES 32