i've just gotten done battling this problem,
for starters sand the diff rings down to 1500, i believe the direction's of about 1/4 turn out is too lose i've been going about 1/8 turn from bottomed out. I'd adjust it down to where it's not slipping then adjust to your liking, too tight and it wants to lock up and you'll notice it driving the track.
i've been breaking my diff in on the bench just took an old dog bone cut one end off and chucked it in the drill.
your stripping idler gears out because of the slipper being too tight, its a fine line between being too loose and too tight. also when you assemble the trans put some shock oil on the gears it'll be much more quite and fluid.
take my suggestion's with a grain of salt it's just what i've learned thus far building my sc10