shimming the diffs mean you add very thin washers to each side of the differential to remove any play in the gearbox.
most differental shims are universal in size, but i'm not 100% sure about them being the right size on the older models..
http://www3.towerhobbies.com/cgi-bin...&I=LXCWP5&P=FR
http://www.amainhobbies.com/product_...Mayhem-Pro-RTR