Block the wheels. Jack up the rear end.
You can just drop the shaft off the flange and it should clear (even on a supercab with the carrier bearing). Use a 30mm? regular socket with the trans in gear.
Pull out the old seal with a puller or screw driver. Clean the surfaces, check for excess abrasions, and install the new seal with a larger socket to tap it in gentle and square.
Throw some blue loc-tite on the flange nut, put anti seize on the flange face (bet you had to remove the d-shaft with a hammer, eh?), and a drop of blue loc-tite on each of the shaft bolts. If it's a yoke style then just clean it out, put new bolts and straps on, and a drop of loc-tite.
The seal will probably leak anyway, but not as much. From what I've gathered, that's just what these T-cases do. Oh, flush and fill the T-case with fresh fluid while you're there. If it has a balance weight on it, the drain plug is under that (which are likely corroded in fairly well) to drain it. Anti-seize these bolts.
EDIT: it's handy to have a buddy standing by to put the trans in neutral or gear. Ideally the whole job would take about a half hour, but with all the possible complications, keep your afternoon free, at least. Might want to consider doing the d-shaft slip joint (lubricating) at the same time for another half hour or so of labor. Somewhere on the forum ADSM did a little bit on the proper lubricant, it's a teflon base IIRC.