Do you have any better pictures of the diffs?The look like alloy cases so probably will be UE Ultralites and 6 or 8 spider internals- 43/13 ratio.
Your plan for the tranny is fine, might want to find a steel idler gear though as the stock plastic one self destructs under brushed power, let alone BL power at this level.
I agree with Scarlettboa about the motor and esc though, thats massive overkill in a truck this size- nothing wrong with low kv & high voltage for efficiency, but that motor is large enough to move a 20lb+ 1/5 scale conversion, let alone a 12-14lb maxx based vehicle. MMM and 1520 on 6s lipo would be rather awesome too, or the 1717- only thing with the latter is its a bit fatter ( huh-ha, it ryhmes! ) so you may have trouble setting the mesh.
Useful tool for working out your speed and gearing, courtesy of BrianG:
http://www.scriptasylum.com/rc_speed/_top_speed.html