Removing the transfer case is not a difficult task, just time consuming. Replacing the rear seal in the 4L60-E in an AWD/4WD installation is a bit more of a challenge, however. I found that mine was installed from the front side of the reverse/OD housing, as part of the reverse clutch hub seal:
You may find that your's is different, but if it isn't, you'll be removing the transmisison and disassembling it to replace the seal.