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T20780 you referenced is the input shaft for the constant mesh transmission, not the HLR transmission. Is this the shaft at the front of the HLR and the splines on it go into the clutch disc? If so it is the input shaft, not the output shaft. It also drives the reverser oil pump. I think it is part of AT21511 assembly they call U-joint for some reason. Read the headings on the pages, your need the ones that say (HI-LO-REVERSE TRANSMISSION) as part of the page heading.
Look at item 17 on this page. https://partscatalog.deere.com/jdrc/sidebyside/equipment/64777/referrer/navigation/pgId/170778
Nortrax may be able to get an aftermarket one and related parts. Other than that I expect it is a salvage yard search. The reverser will need to be taken at least partly apart to change it, your service manual should have info.