My F70 has suffered the same fate - 4 wheel drive doesn't work anymore.

I'm guessing the splines get stripped for several reasons, the main one being maybe due to using 4WD on tarmac, so the transmission winds up (caused by the fact that Fourtraks don't have a centre diff), or maybe due to corrosion caused by seal failure on the sliding part, and the loss of grease / water ingress or the lack of grease due to neglect.

FYI the really early Fourtraks (like mine, upto about Aug 1985?) have rectangular flanges, whereas later ones are square (they are not compatible).

I got a second hand front prop off Ebay for about £20 - it's got square flanges so I need to replace these with my old ones, by splitting the UJ (Milners say the UJ are compatible).......

There are several companies that make new prop shafts, one is in Middlewich / Northwich(?) whom quoted me about £200-300 (2yrs ago) to make a new one. Genuine Daihatsu part was £450 + weeks to import from Japan!