Have you looked into a hobby-grade R/C tank chassis; perhaps something from Tamiya - for instance:http://www.tamiya.com/english/products/56010tiger/tiger.htm
Mind you, these won't be inexpensive if you go for the full kit, but you might be able to get just the "running gear" if you do a bit of research (go to some hobby stores, order catalogs from Tamiya, send them an email (or call them), etc. Basically you just want the bottom metal piece, gearbox/motors, wheels/suspension and tracks. Maybe the ESCs as well.
Another alternative (cheaper) would be to pick up a 1/16 scale (or thereabouts) R/C airsoft tank; these tend to be cheaper quality - you see them all the time on Ebay. Then tear it down to the chassis.
Lastly, there are a few el-cheapo full-function toy R/C (and wired controller) tanks out there, that can make for a good chassis...