I have the same exact problem on my ex wifes 2006 4.0 explorer. 122k miles. She ran in hot for a day without telling anyone, running it until it cut out on the highway. In the past week, I have replaced the thermostat and the cracked radiator. Now when I go on short trips no problem with overheating but Iam noticing coolant in the block cavity behind the thermostat housing. I think the leak is coming from the rear of the housing, but I took it to Pep Boys yesterday and they ran a pressure test. They said that they ran it for 10 minutes and nothing then after that they had a positivie test for hydro carbons in coolant. They said it was a blown head gasket but they did not sound real sure. I also have a high reving sound when i accelerate. No milky oil, no smell from exhaust. How did your situation turn out on your truck??? please advise, I was going to replace housing but now after pressure test I am at a cross road. Pep Boy master tech said it was weird because normally it test positive right away and mine took 10 minutes of running.