If you didn't know already, you can look up all the parts online on agco's website:
SSO
Enter 1440 in the search bar, then pick whichever is your tractor.
It appears there is a splined shaft and two couplers. No u-joints like a typical driveshaft, so I don't think this is the kind of thing you can have a normal driveline shop build or rebuild.
BTW, I can't see the driveshaft on my tractor at all, so are you just guessing what you broke, or have you already disassembled some? Lots of places for 4wd to let go that aren't the driveshaft... ??
And if you don't have a local dealer, or you want a sanity check on pricing, you can buy anything still stocked by Agco from Jackssmallengines.com. Just enter the P/N to their search bar and it'll find it.
FWIW, the forward shaft appears to be available for $200 on Jack's site. It usually comes back as not available when it's not available...
Good Luck!!