Aluminium would indeed be a bad upgrade choice, not sure what the stock ones are- probably just unhardend steel. Try and find some hardend steel versions from.. somewhere ( hpi forums may have some more specifc info or leads ).
I was having the same problem with my savage and the hd drive gear solved the problem. The stock gear is unhardened steel. The hd gear is hardened steel and will last longer than the rest of the truck.
They say a good mechanic only needs 2 tools - WD40 & Duct tape. If it moves, and its not supposed to, duct tape. If it doesn't move, and its supposed to, WD40.
So the hd drive gear + the high speed 39t idler should result in a "bullet proof" center gear box locked in 2nd, right?
The point about bearings is certainly a valid one... Ive had more bearings than anything else fail in my truck. Now Ive made it a habit to swap bearings at a regular basis...