Thanks for all the tips guys.
I opened the diffs again to check the shimming, and they seems fine as they were.
One problem though, the front diff had come completely apart, would this just have been a case of me not tightening it enough?
I followed this page of the manual
http://www.oople.com/rc/manuals/zx5/...s/zx50006.html
but I got a little confused by what it says. It says to use two screwdrivers to lock the two shafts, then move the gear, then tighten the nut and repeat. It says it should be tight, when it means tight does it mean you shouldn't be able to move the gear, or does it mean you should be able to move the gear but it should be difficult to move?