I would start off with the fuse box, or did you check that already? The diagram below is for a later fuse box but you should find you have a fuse for those circuits.
If it is not the fuse you may have to start from the beginning and check everything. Have you got a test light? They are only $5 and will save you a lot of time and effort. A meter is also very useful in this circumstance, as well as the usual electrical tools and crimpers and a soldering iron. Most of all get hold of a proper wiring diagram for your year and identify which wire is which and where they run. www.offroaders.com
have some but may not be right for your year.
First off, have you got voltage at the fuse? If so then it is going to be in the wiring or fittings back to the lamps.
I suggest you start with establishing what you have at the tail lights and what you do not have, electrically.
Stop light left
Stop light right
Reversersing lamp left
Reversing lamp right
Side marker left
Side marker right
Once you have stablished if each one is dead you then need to get the fitting apart and check for voltage at the fitting on each of those circuits.
If no voltages go back to the plug directly behind the fitting and check there. There are only 3 prongs on this plug but there are probbaly 4 wires on one and 5 on the other as the tail lamps and side markers are spliced together at the plug and there is one splice on one side of the backup lamps. Your wire colours at these plugs are:
Green with tracer - turn and hazard left
Green - turn and hazard right
White with tracer - back up lamp
White - tail lamp
If you look back at those photos of those plugs you can see those colours. Take the plug apart and check the prongs for voltage. I would suggest concentrating on the white wires as this appears to be your problem.
I would then go under the dash to the frame connector plug. These wires run down the side of the tub in a small loom to a plug under the dash. Follow that loom until you find it. You will find a pink wire in there as well which goes to your fuel sender. When you find the connector it should have 5 prongs on it.
Test at the connector for voltage on the white wires. If you have nothing here you know the problem is further forward. I would then suggest going to the lighting switch and having a look there for voltage. You may have a burnt switch.