RE: '98 Durango overheating problem
I have absolutely identical symptoms as you, except no power loss and it's on Ford 351 CID. On thing I did was do that you didn't yet is do a block test (from NAPA) to chemically check for combustion gas in the radiator. Instructions say the blue test fluid will turn yellow if the combustion gas is present. In my case it turned green and I'm certain there was no green antifreeze sucked into the tester. No mention of what green means except on a diesel engine. A shop then did the same block test and said there's no combustion gas leak, as test fluid remained blue for them. Lower portion of radiator is 60-degrees F cooler than top half at normal operating temp.
No coolant loss, no moisture out exhaust, no coolant in oil, no oil in coolant. Replaced temp sender, thermostat, hoses, fan clutch, 5K on OEM water pump. Coolant is very clean and new, radiator 4 years old no corrosion inside. A/C condensor front is clean, no flow blockage. Have not done a leakdown test, but I suspect if there is a combustion leak it may not show up unless at full temp, and I don't see how it's possible to get in there to do a leakdown test at full temp.. Have not flow tested radiator yet, tested cat, or removed trans pan.
Did you ever solve this? If so, I'd sure liek to know what it was.