So I crawled under my 99 4x4 looking at taking my exhaust off to make way for my new 4inch straight pipe and noticed at the back of the transmission where the transfer case bolts up is wet. I’m assuming the rear seal on the tranny. Should this be fixable by dropping the transfer case?

Yes sir it is. Get both seals the rear seal for the transmission then the front seal for the transfer case. 

 

Be aware... I've done this on another truck and the seals didn't last at all. Make sure the bearings on both ends are in good shape. If there is shaft movement, grooves in the shaft seal area or damage you might have a leak till you make the proper repairs.