I'm inclined to go wiht a syncronizer problem, too. I've seen SM420s break the syncros for 3rd and 4th gear before. Not too hard to change.
If you do decide to overhaul it, I'd suggest using Lucas Oil Stabilizer as an assembly lube, then run about a 25/75 mix of the stabilizer and Lucas synthetic gear oil in it. I did that wiht my 3 speed overdrive. Made it shift real slick.
PS for the tools, check with OTC or Snap-On. The may list them, and will say in the tool description the equivelant for the factory tool numbers/
