Things like vehicle weight, motor torque, transmission type, gearing, and weight of right foot need to be factored in. To people replying saying their friends run 37s on the D30; do you have rocks like JV? If not, shut it.
I think Richard's (goatman) XJ can be used as a good point of comparison. It's heavy, has a stroker motor, and fairly deep gearing (also an auto, which is generally considered to be easier on axles). He runs 37" MTRs, which are basically the same size as 36" iroks, on the same trails, and runs a front D44. He occasionally brakes hubs, stub shafts, or CTMs, but I know he has attributed much of that to residual effects of towing a dead XJ out of much of Fordyce. He is also frequently talking of upgrading to larger outers, but the D44 stuff seems to get him by.
take that for what it's worth.
my opinion is that if you can keep your XJ under 4000 pounds trail ready, run a minimally modded 4.0, auto tranny, and nothing crazy for gears, you will be fine on a D44 with alloys in the front and a 33 spline D44 in the rear. however with this combination still be prepared for a very occasional break, if you're looking for bulletproof you need to go 1.5" front and rear (60s, 9s, whatever)