Agree it sounds like a fan problem. A new T stat wouldn't hurt. I guess if the T stat was only opening partly, it may be enough for the highway. I'd go ahead and put one in. I'll add its worth triple checking coolant level in the radiator after its all cooled off first.
As a diagnostic, can you watch what the fan does while hot? Run it and let it idle until the guage climbs then feel for air flow in front of the radator. Pop the hood and see what the fan is doing also.
FWIW I changed the clutch on my 2006 Durango at about 130k. Apparantly the factory installed the clutch with locktight. I had to make a holding tool to get enough leverage to remove it. I also tried a "torkflow" brand clutch from AZ. No good, it howled like an air boat in winter. The replacement from MOPAR worked fine.