How hard is it?

What's used and for sale now is not really promising. I'd just convert what you have - it is cheaper as you can leave the pinion gear in place if it's in good shape now but just a one-legger. I'm not a fan of the Sure Grips availabel now either. Buy a Tru Trc, stick that in with new carrier bearings, and go. If you want to change ratios you can do that. Cost in parts is about $700 + S&H for the Truc Trac and bearings; about $1000 + S&H for ring & pinion, Tru Trac, and a few other "should haves". Labor can vary, I get $200 for regearing them. If you don't have the tools, prob better to have a place that does 4wd work do it.