This is always a good subject for a debate. I'm on the lock the rear axle first side. My reasons are that the rear axle is usually stronger than the front and when you are climbing the weight is usually shifted to the rear tires.
'86 CJ7, 258 w/Mopar MPI, 4.56 gears, rear Detroit/front Truetrac, full cage, OBA, 8274, 4" lift via Alcans/Rancho, 35's on beadlocks, other stuff. Built for rocks, mud is a four letter word.