Pull the foot off the leg and see if any shaving fall out. Mine had been binding for a long time, same symptoms as yours. About a month ago, I was raising it off blocks under the frame and the whole drivers side dropped 4 inches.
When I took the foot off, a bunch of shavings from the worm gear came out. Evidently something (bolt/washer/nut??) was stuck in there, eating away at the gear every time I used it. Ended up putting in a new leg and it works like a champ.
Like others said, inspect the top gears under the cover. If they are chipped or worn, you can get a replacement kit from Venture Manufacturing or even cheaper on the internet (less than $60).