Even if you couldn't there are places like RC Alloys that make custom driveshafts. Matter of fact, I have one on order from them now.
Now I'm not a big crawler guy so I can't speak much of the chassis design and suspension work, but my only issue with what you have planned so far is your powerplant. IMO, anything aside from a 4-stroke is a mistake. The FS26SCII is actually a very compact engine that offers MUCH more torque and consistant idle. One of the big advantages of the 4-stroke aside from the torque is it's ability to idle without "loading up". So if you need to just stop and think about your next move you don't have to worry about the engine loading up with fuel (as nitro 2-strokes do) when you get back to the gas. A nitro will want to bog if it is allowed to idle for any extended amount of time, this will make your re-entry to the throttle very inconsistant and the power will be unpredictable. It's not an extremely expensive engine either.. $200 at Tower.