As previously mentioned, bad fan clutch. When the rig is fully warmed, the fan should be blowing strongly. I had an overheating problem with my Trooper and it ended up being the fan clutch.
There are a couple of brands listed at Rockauto.com, as I recall I got a Hayden clutch for my Trooper.http://www.rockauto.com/en/catalog/isuz ... lutch,6812
Found one on eBay sold by Pep Boys for $40.68 shipped. Which would make it a better deal than Rockauto, where the prices are Plus shipping.http://www.ebay.com/itm/Hayden-Premium- ... t~&vxp=mtr
Also do not discount the possibility that the radiator could be partially clogged and not giving full cooling capacity.
Might be OK for normal, but not for higher-stress 4-wheelin'. You can pull the rad and have a shop do a flow test on it. Replacement radiators aren't too expensive for your rig (under $100) but you're gonna get plastic-tanked, aluminum construction. The aluminum rad likely will transfer heat better than the old brass radiator. http://www.rockauto.com/en/catalog/isuz ... iator,2172
Sure doesn't sound like head gaskets to me, if you can drive it around town without problem.
I'm assuming there's no coolant in the oil?
You can do a test for combustion gasses in the coolant, with a special test fluid.
Test fluid at NAPA: https://www.napaonline.com/p/BK_7001366
To do the test, draw a coolant sample out of the radiator, add test fluid as directed. Turns yellow in the presence of combustion products.
HTH & G'luck with the repairs..........ed