Well, the differentials hold up very, very good. (k4.6 has got 6 spider diffs) to take these diffs to a higher level, you can buy two HB lightning diffcups (5 dollars each) and 2 pcs 8X16X5 bearings. (those cups can use a larger bearing, beacuse the output side is from metal instead of plastic, and besides that, the plastic HB uses is much tougher than the HPI plastic. (keeps your pins from eating thru the plastic, though with the power you'r going to put in, you don't need to worry about those diffs.. The 10L is a perfect motor to keep your truck going and going and going, without servicing it too much..
Only thing you need to consider, is an aluminum differential case. this is the real weak spot. but then again, the 10L is more gentle on the drivetrain, so you should be fine, but if you hit the brakes too hard (and the K4.6 includes the HPI dual brake system, which brakes VERY hard on a weak servo allready) the front ringgear and pinion can be shred from this braking. (the aluminum diffhousings prevent this) i know a person who runs his savage with a 10L and he races it very, very much, and he never wasted a ringgear and pinion. I also heard that it was likely the first batch of these pinions was weak. (and i only wasted the two stockers, and i bought my first SS 1,5-2 years ago now)
Only company who makes these is GPM, did you see
this thread? If you can lay hands on L-profile aluminum, you don't need a vice either.