First, thanks Mike for help in trying to diagnose problems. As always, a wealth of info and goodwill. Ended up taking Disco to shop because I just didn't have time.
Shop said radiator was in bad shape, so replaced radiator, throttle gasket, and water pump. They also did a chemical test and determined that heads weren't perfect, and were beginning to seep, but were not thought to be the cause of my overheating under load.
After those repairs, rig still behaves the same; temp is fine at idle, but as soon as its put under load, overheats within minutes. Shop says its a sleeve or crack. Basically have a boat anchor as Mike says.
So now what? Absolutely love the Disco, its my daily driver. Intended to drive it "forever". Quoted a rebuilt engine out the door in the $4K range. Its 2001 w/ 138K miles. I have asked shop to assess condition of everything else. Speaking on pure practicality, I need a car. Is it worth putting in the $4K? Where do you get a used vehicle for $4K that you absolutely love? If a perfect world and installation of rebuilt unit went well, any idea what I could get out of the vehicle going forward? Lots of questions. Heartbroken. Welcome anyone's advice. Thanks everyone.