Hydraulics work well for these machines except when it is cold. Even winter grade hydraulic fluid is too heavy for the first few minutes of operation at over -20c. Magnetic heaters help some, but it is still a pain...
I've enjoyed reading through the information on this thread, and finally joined up so I could comment.
Any updates from the OP or the others looking to build one of these? As others have stated, we'd like to see pictures!
I also thought I'd share a couple of my projects: one is my snow dozer that uses zero-turn hydrostatic transaxles, Miller Snow Dozer - Kustom's by Kent and the other is a larger tracked vehicle I'm building for a client that uses a diesel engine, pumps, and hydro motors, and is also radio controlled, All Terrain Track Chair (ATTC) - Kustom's by Kent
Hopefully that information is useful to people working on similar projects.
I gave up on the idea, life, heart attack etc. Have no desire now. It has been done however if you search the internet enough or u-toob you can find some with basic auto/truck differentials in them and doing great.
If you build one keep us posted