on the diff, I have never replaced the washers, although i guess they would get weak over time. many racers will tell you the shuie diff is the best ever, but for some reason my diffs when new feel like there is some sort of drag, almost like the rings are rubbing on the diff gear. I find using assoc diff lube the red label cures this, however if i just leave the diff with shuie grease alone it starts to feel better over time, with a 17.5 my diffs last forever, with mod, only a few runs before they get crunchy, most of the time its the thrust bearing, I would buy new rings or at least flip them over if you haven't already, diff ring prep is the key to a awesome diff.
on the NW thread under set up tips theres a post with great info on diff building. either that or pay JB to build one for you.
for the axles, I've never trimmed mine, but if you were going to do it, i would say just use a cut off wheel but do it with everything together. after it was brought up recently, I've been looking for something to just put over the end that would sit flush, like a cut down antenna tube cap, or a piece of shrink wrap or a purple nut thats been rethreaded, something like that, but I haven't decided on just what, most of it so far just looks "hokie"