Re: to lock the front or the rear?
Lock the rear first - a friend of mine put in a lock rite in his front D30 when I put in a spool in my rear D44. Same trails we travelled and I could make it up sections he couldn't. Why? Going up hill the fronts have very little weight to pull you up, but the rear carries the burden. His fronts would spin along with his peg leg rear and go nowhere, while I idled up the same obstacle. If you are worried about on road/street driving, go ARB or Ox - selectable when you need it.
Also, a front locker messes with steering off road, especially if you don't have PS, but even then can be a prob.
Good luck with what ever you decide to do.