98 GMC Sierra four wheel drive and over heating problems

[Miss'Behavin]

I have a 98 GMC Sierra 2500 5.7L V8 engine. Anyway, long story short, I was forced to let a friend of a friend use my baby to transport dirt fill for horse stalls and I get a phone call that they blew my radiator. It was over heating before but I drove it locally, and the dirt fill was local so I said shit, why not? Whatever they did is beyond me but i just got my radiator replaced and not only is it over heating worse but it refuses to switch out of 4 HI. I'm not stupid, i know to put the truck in neutral, then switch to 2 wheel drive. I have done that over and over. It's staying in 4 HI and refuses to leave it. So next, my over heating problems, I have now replaced the radiator, bypass hose, thermostat and water pump all in the past 6 months and the truck is STILL overheating. When I bought it, the guy said it only needed a new radiator, my mistake but whats done it done. I just really don't know why its still over heating