Try here for a bit of diff advice...
http://www.thard.co.uk/building-a-trf419-diff/

As for the shocks, I personally prefer the double o-ring route.

Use 2x kit o-rings in each shock bottom, and you'll need to use the original version bottom guide available in 53574 (or cut off the spacer section from the kit ones... not advisable!)
If you add a 0.2mm x4mm shim in there as well (to help tighten the seal) and you'll get really nice long lasting, leak free shocks.

HiH
Ed