I'd at least pull the sheet metal sump and have a look at what's in there - that's not difficult at all and it'll give you an idea how rigorous a cleaning the oil system might need.
On your oil flushing question, I've installed prelube systems on my UZ's, and a modified version might work for you to flush the oil system.
Basically you could get a remote oil filter adapter for the UZ, then hookup a ShurFlo pump and an external inline filter to the two ports, fill the crankcase with diesel, or some other mild solvent and circulate it with the ShurFlo pump to flush the system.
The one unknown with this rigup is whether the ShurFlo could "pull" solvent from the sump, through the existing oil pump to its suction. I have doubts whether it could, so if this didn't work, then I'd put an external "sump" next to the engine (basically a 5 gallon jerry can that you could feed a suction & return line into, then I'd hook up a line from the drain fitting of the UZ's sump to the jerry can, and a suction line for the ShurFlo to the jerry can, then I'd plug off the side of the remote oil filter adapter that goes back to the pump, fill the jerry can with solvent and circulate away...... To guard against cavitation and ensure good priming of the ShurFlo, I'd make sure the engine was higher than the jerry can, and the jerry can was higher than the ShurFlo.
Once I was happy with the flushing process, I'd substitute regular motor oil for the solvent and flush awhile with that so that all the bearing surfaces in the engine would be coated with oil, not solvent.
Note, you should not crank/turn the engine with the lightweight solvent in the system, as this would ruin your bearings.
You'd also want to have your heads back on, with valve covers, before doing this flushing, as it might get a bit messy otherwise, unless you plug off the oil feeds going from the block to the heads