Heres a little how-to on the viscous diffs. The reason the spider gears crap out is because of the o-rings that seal the output shafts. If you take apart the diff you will notice that the output spider gears are keyed to the diff output by means of a small pin, like how a hex is keyed to an drive shaft. The problem is that the thin steel washer that backs the pin is directly on top of the o-ring that seals the fluid in the diff. When the diff spins fast the thin washer is pressed on by the pin. The washer is too thin to stop the pin bending it, and when it folds up it cuts the o ring and lets the output shaft disengage the pin from the output spider gear. My fix was to remove the o-ring and repalce it with steel washers, this also allowed me to shim the spider gears so that they would mesh correctly. Of course this required the use of grease instead of silicone fluid since the diffs were no longer sealed. The washers I used were rc18t wheel washers, linked below. I used 2 per side of the diff to replace the o-rings and it set the mesh tight, but after about 5mins the gears wore in together and I never had a problem again. I used tamiya ceramic grease to lube it up.
http://www2.towerhobbies.com/cgi-bin...?&I=LXJHR8&P=7