Transfer case oil leak, the most likely suspect, the intermediate shaft "O" ring seals, the shaft wears in the bore of the case, "O" rings harden, oil leaks usually forward in the space between the gearbox and transfer case. Removal of transfer case from the gearbox is necessary to fix, but generally not a too big job unless the shaft bores are flogged out in the alloy case.
I have had the same leak on both my manual Disco's, but one was exacerbated by a blocked breather from the Transfer, something you should check, check both gearbox (assuming it's manual), and transfer case breathers are open, pressure can build up in both if not vented and cause excessive leaking and premature seal failures.
