Diffs, there is no right or wrong diff to use as converting the 250 to RC you really are re-inventing the wheel, every conversion has been done different such as the German chap with his chain drives to the sprockets, the best diff would be a heavy duty diff with a 1:1 ratio to simplify working out final gearing, just to make things more complicated you could use
http://www.ebay.co.uk/itm/HPI-RS4-RT...wAAOSwvgdW4HHw
Or you could use
http://www.ebay.co.uk/itm/FTX-Bugsta...MAAOSw~bFWL5~x
Or you could use a pair of MFA motors with planetary gears mounted direct to the sprockets similar to what Tim did with his
http://thepanzertrap.com/panzertrap_250.html this method allows independent control of the tracks, BennyB has some good options and as he says the R2 transmission is slow its primarily made for rock crawlers, all this information is going to sound confusing as there are many ways you can go with this conversion.