You unbolt the seats from the floorboard, put it on top of a blanket upside down to and see if there's anything wrong with the rails.

You can also try white lithium grease on the rails (after you've cleaned it of course) to see if crusty old grease was the problem to begin with.

FWIW: My seats on either driver or pass side make a weird whahhh whahhh whahh noise when I move forward and backwards lol. FL's hot, and I'm just too lazy to go to the garage and take care of it right now