Checked for something small like a 1-2mm piece of a pebble or something sitting on the toothed shaft that's now caught up in the worm gear grease that's getting pushed into the gear-mesh when enough grease has allowed it to be pushed into things yet?
It could also be:
Chipped worm gear or damaged toothed shaft, from something currently (pebble, fossilized Cheerio) or before.
Bad gear lash. Loosen the 4 bolts to the cab and see if it goes away.
(Either way you may want to do this step and then remove the seat to better check the worm gear and seat-sliding rails for that pebble.)
Something stuck/trapped in the seat sliders that's catching and breaking loose (pebble logic again). Basically, bust out a flashlight and/or coat-hanger wire and see what if anything is gumming up the slider tracks.
Good Luck, seriously though look for this stuff. It's not uncommon, I've seen it happen with a BMW and Toyota seat track.
[Edit] See attached video-still pic I got from blowing up and looking closer at your video. My bet is on the logic there's something in the seat rail channels catching. CLEAN YOU TRUCK MAN!
