To test the lockers you just want to see if they engage. Is best to do on gravel or similar, but if not it's no great worry - just try and drive in a straight line if on tarmac.
As far as I'm aware you simply cycle the F+R diff-lock switch round to engage them. The diff-lock lights on the dash will start flashing and will then go solid if they engage. You sometimes need to drive the truck forward a bit to get them to engage. The F+R diff locks will only engage if the centre diff is locked. This may happen automatically when you turn the F+R diff-lock switch but if not simply engage the centre diff-lock youself by either pressing the button on the dash (older trucks) or engaging low range first if there's no button (will then engage centre diff lock automatically.
If you're on gravel or grass or similar and the lights on the dash keep flashing (i.e., the diff-locks are not engaging) driving gently in tight circles can also sometimes help.
It is not unusual at all for at least one (usually the rear) to not engage if they haven't been looked after or used regularly. Sometimes sorted through a strip and clean, sometimes is an electrical issue, though may mean a new actuator.
Just got back and didn't buy it. Very glad I asked about the lockers, because the centre locker seemed to work but the front and rear locker lights wouldn't go solid. All round it wasn't in very good condition either - lots of poorly touched up paintwork, oil/paint spilled on the carpet, hole in the drivers door card, etc...
Non functioning lockers are a common problem with the 80, when I purchased mine neither the front or rear worked, I engaged them on gravel and drove very slowly in tight figure 8 patterns and kept switching them on and off.
Maybe 5 or 6 minutes later one came on and then the other followed a minute or so later, now I take them out for some excercise on the beach once or twice a month and all is well.
I was wondering whether this woulkd work for me because I've never been able to get them to engage, maybe the tight figure of eight for a few minutes might be a useful exercise for me to try too.
Yes I would give it a go, I would also be operating the switch on and off in an effort to get the motors turning. Make sure your figure of eight is driven nice and slowly to allow the teeth to line up.