Your latest info says to me, that air flow is the main issue. Highway will have much more air flow than stop and go...where the fan is the key to keeping air flow up there
Even if you have an El Cheapo radiator, running with just the driver, it should not behave like that...heating up in stop n go, as you still have the external aux ATF cooler
Since a 2500, EVERYTHING is sized for HD usage. Thermal abilities are one of the main attributes making your truck a HD towing vehicle. The margins (head room) are there to absorb that kind of short term usage needing more HP in spurts (acceleration from a stop)
Hope that is all there is, but you never know until the situation is completely fixed.
