Outer Limits 4x4 Board

Hi all,
We have a 1982 suzuki sierra that has suddenly started getting stuck in 4th gear.
When it's not running it moves in and out of gears smoothly and when it is idling with clutch in it moves betwwen all gears fine. as soon as you start driving and get into 4th, that's it - stuck. We can drive it if we only shift up and down between 1st,2nd,3rd and reverse.
I was wondering - we had to get front & rear unviversal joints replaced on rear tailshaft for rego and less than 3 weeks later it starts sticking. Could this have anything to do with it? they had to try 6 sets of universal to find ones that would fit because it's not the original gearbox.
The only other thing that has been touched is the drive shaft from gearbox to transfer box had to be removed to replace the input shaft oil seal. We are selling it and want to fix it for least amount of $$ someone suggested thrust bearing? we don't know what to look at first. If it was the locating bolt or something would it still move through the other gears ok? please help we are on holidays so hubby can do some mucking around if given any ideas.
Thanks heaps.
Jo

I would start by checking the oil level in the box. If this is OK then problem is most likely a worn shaft/bearing in box allowing gears to 'over-mesh' & lock. Rebuild kits are available readily and fitting instructions can be found on the net.