I have thought about a custom chassis actually (great minds...), only issue is making it; I would probably get the Candyman to make me the plates as he's cheaper than a proper machinist, and has a flair for this sort of thing too- even then though, I cant see it being cheap. What Im probably gonna do it just use longer cantiliever push rods- doesnt need mega suspension travel or articulation, just enough to be 'useable'...
As for gears = no, saddly. I've seen someone make a custom spool that uses a larger 'ring gear' and smaller reduction gear thingy above it, I think using a pinion on the shaft and a delrin ring gear, but that was in a custom crawler to get lower speeds. Trouble is really it uses too many gears in the axle- cant just find a new 1/8 diff to stick in there, got to replace the upper reduction gear and the two pretty small bevel gears that change the drive direction 90*.
Ultimately, if the gears in the axles prove to be too weedy, then I will probably pick up something from rc4wd.com, they have a wide selection of aluminium axles that use the simpler ring & pinion gear design, much more durable in theory.