Looks like Nate2002 hit the starter problem on the head. Sometimes it is the simple stuff. I've never had an 81 to wrench on, I thought there was only the clutch switch and the brake switch(s) had nothing to do with the starter. Those 81s changed a lot of stuff from what I'm used to, so it wouldn't be a big surprise.
An 81 should have electronic ignition, so no points and I think no ballast resistor. I agree it does sort of seem like a weak spark but the electronic ignition is a little tougher to diagnose, not to mention more expensive to throw parts at. A test light can still tell you a lot though.
It could be a fuel problem as well. If you can see fuel in the sighting window on the front of the carb it should have enough to start, but the carb could be plugged somewhere. Splash a little bit of gas down the carb and see if it fires over. If so, start looking at the fuel system. I can't recall offhand if the 81s had the mechanical pump like later Isuzu PUPs and troopers (sits right above the distributor if its there) or the electric. If you have the electric, the relay that runs it seems to be a common point of fault. You could hard wire the pump to the battery to check it out. If you've got a mech pump, check the output by unhooking from the carb and cranking it over with the hose in a bottle. If it puts out much at all in 10-20 seconds of cranking, it should pump enough for the truck to start and idle at least.
After the last fix you found, I guess it goes without saying that I could be wrong.

I can't think of anything simple to look at, but I'm sure there is a relay or two in the ignition system and connections all over that should be checked. Its hard to get the timing very good just cranking the truck over. However, its worth hooking a light up and checking it if you have a buddy or a way to crank the motor over while you check the timing. If it is far enough out of time it could keep it from running.