[JimmyD]

The fan is controlled by the clutch which reacts to temperature changes.

I too have an LMM that I have towed heavy with in the mountains. The difference between our trucks is that my fan clutch engages between 210 - 220. And, there is no doubt on when it engages based simply on the noise that it makes once engaged.

If I were in your situation and with the knowlege that the stack is clean, I would replace the fan/clutch assembly with OEM.

If the fan does not lock up/engage when the engine and outside air is cold, that is another indicator that the fan clutch may be bad as well. Mine locks up when cold and remains engaged for about 1 minute here in FL.