You don't actually mention that the wheel bearings are all on the same corner, but I assume they're in the same corner?
I'd check to see if there is something wrong with the axle stub, maybe buy another axle stub. If there's something wrong with it then the axle nut may be bottoming out on the axle stub or something like that before the bearing assembly is properly preloaded. If the axle stub was stripped you'd likely have serious problems getting to full torque, so you'd know that.. If the Jeep had a decent amount of mileage when you bought it, and a wheel bearing went bad, it's possible that someone damaged something replacing a wheel bearing before you bought it.
Also, what are you doing when you torque the axle nut? Are you lowering the chassis back down to hold the tire/wheel stationary while torqueing it? If you're doing that, you may be putting too much load on the bearing before preload is applied, which can be a damaging thing to do to a bearing.
But, if you've over-torqued the bearing, that would kill it pretty fast as well. Probably not as fast as if you're totally lacking preload though, hard to say.
I don't think you're failing bearings because the bearings are bad, I think you're failing bearings because something is wrong with the other parts or with your assembly method.
Once you find out what's wrong, you could also try Timken or SKF units instead of Moog, especially if there is a tapered roller bearing unit available. I don't know why Moog bearings would have problems, they make really good suspension stuff but I've never used their bearings. Timken and SKF are all I try to buy usually.
