Have a Craftsman 315 series 2 HP motor unit. My unit's depth adjustment froze up too. After hours of fussing with it, I decided to disassemble the entire unit(was off warrantee anyway). Removed the motor assembly, switch, brushes so all that was left was the body. Came apart easily & not complicated, just took care not to crimp or pull on the wiring.
Set the base in a padded vise & gently but firmly worked the unit until it started moving. I did use a silicon lube spray that dried quickly but left enough of a film to make the unit slide a little more easily. Like BJ & Dutchman suggested, blow it out thoroughly with an air hose. Again, wiped off any residue & it cycled it up & down a few times until it moved like new again. Did notice that the "stop" peg in the groove gets hung up easily & care should be taken to make sure it's aligned. Reassembled the unit, a total of ten screws, four in the motor, four in the cover & one each for the sleeves that hold the brushes. I'm more shocked than anybody that it works like a champ again but will use my 24 year old Black & Decker router with a base plate adaptor on my Craftsman router table. Just use this router for regular duties. Other than this problem, it has always done a great job for me.