Sounds like the ball bearing that holds it in isn't there anymore.
If you pull the end cover off and remove the viscous coupling you will see 2 holes in the spline that it came off. There should be a ball bearing in one of those.
doesnt really look like the front has been off recently. though when they removed the box from the car they didnt remove the reverse switch, because its fucked hahahaha
you will be able to use the ball bearing from your other box too, its a bit of fiddly job, i found those magnetic parts grabbers are great for holding the BB while installing etc.
I'm worried now that the ball bearing has been hopping around in there damaging stuff. I won't be happy if I put in the box and it's fucked ... I suppose it will leak if I just put it in as is, or the ball bearing will lunch the box.
the ball bearing cant just fall out/move, unless the output shaft and centre diff inner spline are serverly worn.
it may leak, it may not, once you bolt the transfer case into position it will stop the shaft fallling completly out, but it may move enough to leak.
if the BB is floating around the bottom of the box somewhere, its just a lottery. you may win, you may not.
I agree that the ball bearing locating the output shaft cant "just drop out" unless something is SEVERELY wrong. It is most likely that someone just hasnt put it back in a previous maintenance job on the gearbox.
The ball bearing is easy to get from bearing places. I get lots of boxes coming thru the workshop where the ball bearing is mising. Ages ago I stocked up on replacements... just ask for a 1/4" ball bearing ... easy !
If the ball bearing is in the gearbox hopefully the magnet is in place and would have grabbed it . Otherwise, it would have caused big damage ages ago, and as I stated earlier... the ball bearing cant just drop out from the shaft !
Some interesting info in this thread........Hope it all works out for the OP too.