Matt, it's really easy (compared to say, replacing the seal that goes between the tranny and t-case ;-). Unbolt front half of rear driveshaft, leave the diff end bolted. Swing it out of the way or bungie it up. Older rigs have cotter pin/castle nut, newer ones like your 77 will have a stake nut. You really should use a new stake nut, but the old one can be reused in a pinch, just not recommended. Nut is 27mm (I think) on yours, my 71's a 29 which I didn't have but 30mm worked.
As has been written, drain the t-case first ;-)
After the nut and washer are off, you can use a seal puller that looks kind of like a claw hammer. Just get in there and yank. You can get more clearance by taking off the e-brake by unbolting the 4 bolts that hold the backing plate to the t-case, but it's possible to do it without removing the e-brake, just more difficult clearance for the puller.
Ige's idea is a good one, but more stuff has to come off. In any case, since the shaft sticks out so far, you can't use the socket method of installing the new seal unless you take the speedo housing off as Ige suggested. I just tap carefully around the edges with a lead shot filled rubber mallet. Keeping the frame square is the trick. Smear some grease onto the lip of the new seal before you put the e-brake drum back in, and don't forget to use sealant on the splines of the output shaft when you put the e-brake drum back on!