Hi Cruiser larry:
If you go under your truck, you will see the engine oil pan and the drain plug for it (if you're head is towards the front of the truck and you are on your back). Just slide yourself another few feet back and you will see the another drain plug before you get to the Transfer case hidden under the small, skid plate. The second drain plug, towards the back, is the tranny drain plug. It's the same size (IIRC) as the oil pan plug. Unscrew it and let it come on out.
NExt, if you open your hood, you will see the transmission dipstick. This also acts as your fill plug for the tranny. Get a long fill tube or funnel and stick it in there and refill from this end.
Now in terms of how much comes out, there is some disagreement on that, but generally you get about 4-5 quarts of fluid on the drain and you should refill the same going back in.
I would recommend that you check out this link for more info.:
https://forum.ih8mud.com/showthread.php?t=19299
Also, do an search in the 80's-tech forum and search under 'transmission fluid' or 'tranny exchange' etc. to get more information. The search function is really quite nice here and tranny debates, converstations, etc. have been kicked around for a long time with some really good solid information. Many live by search to cut down on the same questions being asked, over and over again. It get kind of tedious retyping information in that is readily available on the boards.
I use Mobil 1 tranny fluid--you can get it at most auto stores (depending on where you live) IIRC, about $6 per quart, more or less. If you are going to synthetic, I would recommend this as would many other members of the forum (other use Amsoil, or Redline too). Also, you will want to do this a few times for a new to you cruiser. For example, drain and refill the tranny fluid with every oil change (say 3K for about three or four oil changes) and soon, all of your tranny fluid will have been converted to synthetic rather than dino.
Best regards and welcome
-onur
Akron, OH