its easiest to do it with the box out of the truck, which isn't all that hard to take out, just disconnect the tie rod, take out the 3 bolts holding it to the frame, one bolt that holds the steering column together, the lines, and drop it out the bottom.
I can't remember the exact size of the pitman arm nut, but its big, over 30mm I believe. You'll also need a pitman arm puller, once you've got the puller tight, give the side of the pitman arm a couple good whacks with a ball peen hammer, and it'll fly right off of there. You'll need to take the shaft out by undoing the bolts on the top of the box, then changing the seal is cake. Put it all back together, and be sure to use lock-tite on the bolts that hold it to the frame.