On the diffs, my 1st set I built leaked, and I have found the following to help, my diffs never leak now, all around them dry as a bone.
1st, mine leaked because I used threadlock on the grub screw as per the instructions, I have found that carb sealant works fantastic here, I even will cover the whole head of the allen/grub screw with it so I know its not leaking.
2nd, the diff gasket was leaking on one diff. I asked Degani and he replied they like to be soaked for a bit in the diff fluid, and as well make sure the sealing surface on the diff cup is smooth as well, if not hit it with some sand paper. When you tighten the 4 2mm diff screws, do it as you would a cars lug nuts, in 3 stages. You can give them pretty good torque, thanks to the nice quality of SH plastics.
3rd, the drive cup o-rings. If you have the black ones (some of the pro buggy kits included these), ditch them and get some revised ones that are pinkish red (thats all we sell). I swear by TA's Green Slime, I squirt a pretty decent amount in the area the o-ring sits in, the put the o-ring in, smear the now partially displaced green slime back down on it, then stick the drive cup through, making VERY sure it didnt unseat the o-ring, and then smear the green slime back down making a seal around that area.
Fill the diff cup about 2mm over the top of the spider gears, put the top back on, and voila'! Nice leak free, bulletproof Z-Diffs!
Zero problems with diff failures, thanks to the great metal SH uses in the gears.