Broke the truck all the way down and cleaned everything up with an extra toothbrush (not my wife's ). Worm gear definitely has a crack between two of the teeth - see photos below. I put the whole thing back together, greased it up, and tested it out - definitely still binding
I got a reply from Brasseur/Train Doctor folks and they said they don't know the part number for the worm gear and don't have any of them in stock. I'm following up to see if they have the whole truck instead as I wouldn't be against replacing it if the price isn't too steep.
I could also just disconnect/remove the motor for that truck - 3 motors without binding is better than 4 with binding.
Any other ideas for replacement gears?