I have a strictly off-road, '80 F350 with 39.5" Boggers and every time I've gone wheelin' this year, I've blown either a power steering hose or gearbox seal.  Anyone have an idea what's causing this and how to prevent it?  

Yep. The big tires put to much stress on the steering components. Add either a hydraulic assist or go to full hydraulic steering. I had the same problem on my rig. In fact I ripped the steering box off of the frame wile wheeling in the rocks.  

Ya beat me to it. Ya got to do your homework before ya make big mods like 39 inch boggers. Otherwise ya find out what can go wrong at the worst possible time, and someone will say, "Oh yea, that will happen if you do that!"  I hate when that happens!

Hydraulic assist is the best fix, but be careful on highway use, especially in the rain at speed. Hydro assist can get squirrely on you sometimes. I would not recomend full hydro myself.

Thanks for the response; I figured as much, but I wanted to confirm before I spent the money on hydraulic assist or change to a solid axle and full hydraulic.

Happy wheeling!