I'd blame the radiator first. It is hard to tell if it is clear, partly blocked, or mostly blocked. You can tell somewhat by getting the motor warmed up (t-stat open, top radiator hose should be warm) and then shut it down and feel the radiator. If it has big cold spots those areas of the rad are blocked. It is fairly common, I've had 5 LUVs that I used as a daily driver and 4 out of 5 needed the rad replaced or professionally cleaned at some point.
It certainly is possible the water pump is part of the problem, but it just seems less likely to me. If the impeller came off the shaft or was spinning on it, it could be the problem. I'd think you'd see some signs though like it might act like the t-stat was stuck closed and the top hose would never get hot. Also the heater wouldn't blow much for hot air even as the motor was overheating. Flow to the heater core can bypass the thermostat and radiator so if its not getting hot that would be a sign that water wasn't flowing.
The stock cooling system on these trucks is a delicate balance. It doesn't take much to go from normal working to a steam machine
<edit> We posted at the same time so, ditto to egg's post

Just one thing to add: My preferred filling method is to pull off the t-stat cover and fill it to there, then put that together and continue filling through the radiator. At least then you have a pretty good idea the t-stat isn't holding a bubble under it which might keep it from opening.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
