This is a long shot but might be worth a try.
Last year I bought a 1993 Kubota 4WD tractor. The front differential had a leak at the input shaft. Didn't even want to think about trying to tear it down and repair so I did some research and found this recommended on a tractor forum.
ATP AT-205 Re-Seal Stops Leaks 8 Ounce Bottle | eBay
I drained the old gear oil and filled with new oil and this. The leak stopped immediately and I've put +500 hours on the tractor since.
For the price I'd give it a try.
If that doesn't work you may take the jack off and take it to a hydraulic rebuild shop. Won't be cheap though but probably better then a new jack.