1HZ Troopie Gearbox jumping out of gear

It could be that the gearbox is worn and the extra heat caused it to play up.
It could also be asign of problems with the main shaft and transfer input gear wear which will if it happens stop all forward motion.
the wear on the main shaft and input gear can be checked by removing the PTO cover on the transfer, hold the main shaft (jam it with something, wood etc) and see if you can move the input gear on the shaft, if there is any appreciable rotary movement in the splinesbetween the gear and shaft then you will need to fix it.
At 300k I'd expect a full rebuild on both gearbox and transfercase.

REBUILD TIME NOW...don't wait for it to get worse . you most likely have a worn input gear & shaft 300,000 is a good innings for a L/C box.Cheaper to fix now than when its completely stuffed.

Mine was jumping out of all gears when lifting foot off the go pedal. I put up with it for near 12 months , just had to put my hand on it going down hills etc, until at about 530,000 ....KAPUT

Doug