When you reassemble the gearbox, does the diff assembly still shift side to side? If so, you need to keep working at it and keep tapping on the side panels. If this still doesn't work, then it may be time for new gearbox sides. I've seen several Frog metal parts bags for sale on eBay. Not too pricey.
ORIGINAL: Willybomb
bah.
I spent a couple of hours last night rebuilding and banging the gearbox side panels with a hammer/socket and got nowhere. Runs fine in reverse, now goes forward as opposed to staying still but it still sounds like tearing paper. Any hints?