Hi, I found this description some time ago, but have not done it myself:
Tamiya Diffs:
here are the part #'s needed (y) 51549 x2 / 51554 x2 / 51553 / 54540 x2 / 54533 x2 / 51255 x2 / 42259.. You will also need 16 total 2x6mm screws to hold the diffs together F&R and four 850 bearings vice bushings if you want them for the diff outdrives ... Now for the fun part... you will need to mill 1.5mm off of the Bevel gear side and 1mm off of the other side so that you can make the TB04 gear diffs work in the TRF502x cases... the upper gearbox case has less clearance than the lower so you might need to become a little handy with Dremel
.. but once they are milled and fitted properly.. they are every free