Yes they are rebuildable, and pretty easy to do, but the parts are insane. Plus the older J type is a bit light duty for a big truck, that's the same unit that came stock on the older Volvos and MGs. Last time I rebuilt one was about two years ago, the cone clutches are still like original, a leather band bonded to one side of a cone that goes to the metal side when the hydralics trigger it under pressure.
Figure $4-500+ for the cone clutch, $100+ for the hydralic rebuild kit, $300 for the solenoid, plus a few special tools. All that and it's anybodys guess if there's something completely thrashed in side.
I kind of wonder how trashed the thing is, since you run those things on ATF or real light motor oil and he was running gear oil in it. Looks like a pretty big gamble.
-------Robert