The tail end of the transfer case on my 1990 full size Jimmy was leaking fluid for a while. I repaired it, but I was wondering where to add more fluid, what the fluid level should be, how is it checked, etc...

 

Not sure of the transfer case in your truck, but to the best of my knowledge all transfer cases fill and check the same. Look for two "hex key" plugs. One high and the other low. On my NV 233 they are on the rear of the transfer and use a 10mm hex key to remove.

To check, remove the top plug. Put your finger in the hole. The fluid level should be flush with the bottom of the hole. To fill it I use a siphon with one of those rubber bulbs you squeeze.

If you wanted or need to change the transfer case oil...using a drain pan underneath you would remove the bottom hex hey plug.

 

You should have a NP241C transfer case. ^^^basically correct above except instead of hex keys there will be a bolt, a big one that typically will use a large crescent wrench to remove.

Where was the leak coming from? Slip yoke, tail shaft seal, speedometer?

Note in the following page you don't need to remove the skid plate:
Autoblog Maintenance 101: Transfer case fluid change

 

It was leaking from where the tail shaft section was bolted to the main body of the transfer case housing. I fixed it with the Right Stuff from Permatex.

 

Fill plug is on the back of the transfer case, towards the top. It's a big 1 3/16 6 point socket.