A Tamiya semi axle would be a great axle to go with. It's narrow enough for what you need, maybe a little more narrow then needed, but you could always make your wheel hubs a little wider to compensate. I'm not sure what the differential's ratio is. Been a long time since I had a set of these axles laying around. They have the same diff ratio as TLT-1 axles and use many of the same parts.
There are a lot of popular axles out there you could look at. Axial's SCX10 axles, or maybe their AX10 axles. Might be the same basic design even. Possibily you could consider Axial's XR10 axles too, but those are a motor on axle design, similar to what the Clodbuster uses, but narrower. These might be a little on the wide side, and the differential is non-existant. They have a spool so both tires turn together, regardless of traction. Otherwise, you could also browse RC4WD's axle section. They have a lot to choose from also. Take a peak -
http://store.rc4wd.com/Scale-Crawler-Axle_c_12.html
I think bang for the buck, you'd be best with the Tamiya axles. And we know they are strong and very common, so getting them and parts would be easy.
Another crazy idea might be a gearhead motor that has a 90 degree bevel drive box on it. Two of those could probably fit in the rear of the model, although the motors would be intruding on some of the space in the belly area of the frame.