The gear box input shaft on my 54D MMM started leaking, I removed the seal and found the timken type nut underneath was loose. There is absolutely no information on the gearboxes and only a very few parts available. I managed to get a new seal and made an educated guess what the tension of the nut should be, and fitted the new seal. That was about a year ago, I have had no problems since.
When you store your deck vertically, are you sure the leak is coming from the bearing seal, or could it be the small vent on top of the gearbox?.
If it is the bearing seal, doubtful Deere will repair it unless it is a warranty issue. Unless things have changed in the last few years, the deck gearboxes are not considered repairable items (which is why you don't see much or any parts breakdown info on them). When they fail, it is a complete replacement item.
When I bought my 2008 X748 (new), I had just over 100 hours on it and the input shaft seal started leaking on the 54C deck I had. When I further investigated it, the shaft stub (where the driveshaft from the trans connects to the deck) had noticeable side-play, indicating an impending bearing failure. I took the gearbox to my Deere dealer, who told me it was a replacement-only item and individual parts were not available from Deere. At the time, it was mid-Spring and I had my entire mowing season ahead of me, and the gearboxes were backordered with no availability date. The dealer didn't offer any solution, but after I complained that it was poor CS not to have such a critical part available, he took the gearbox off a new 62C deck in his warehouse and gave it to me as a warranty replacement.
These gearboxes are sourced by Deere and not built by them. Apparently, they feel the labor to rebuild one, plus whatever parts are required, is almost the cost of the gearbox itself (in wholesale terms). A new gearbox was in the $650 range back then, not sure what it would be today.
I can't imagine that a good machine shop couldn't replace the seal, or rebuild the whole thing, if they can match up parts for it. The lube level in the gearbox is just lower than the seal, and the gears run in the bath, so there is no real fluid pressure against the seal if the deck isn't vertical. If your seal doesn't leak when the deck is horizontal, and there is no side play or end play in the shafts, keep using it and keep an eye on the lube level. Save up your sofa change against having to replace it.