The T-case comes stock with a slipyolk. That rear seal is known for leaking.
You can replace the seal, & it will stop leaking. It may start leaking again soon, if the slipyolk guide bushing is badly worn though.
This bushing is in the T-case's tailhousing just ahead of the seal.
I replaced my seal, without knowledge of the bushing. Started leaking about three months later. Too lazy to go back & fix it right, I just top off the case about every three months.
A swap to the slipyolk eliminator, will cure this ailment. I have that on my never ending list.