I had a buddy with a set of the old fox racing shocks on a defender 90.  He ran them for a while and they were really stiff but then sent them back somewhere to get revalved and then they were awesome.  I like the idea of something you can tune yourself.

I've also thought of looking into some rebuildable revalvable circle track shocks.  I was looking in a speedway motors catolog and the QA1 stuff and other dedicated race brand shocks look promising if you could get the length we need.  Kind of unconventional for an off road rig but the parts to revalve are super available.