2005 Dodge Ram temp gauge goes up, just below the red mark

While in 4 wheel drive on the beach my temp gauge climbs to just below the red mark. I don't want to overheat to the point of messing up the engine so I turn on the defrost (heat) to lower the temp. What can be causing this? I have lowered my tire pressure down to 20 psi to get better traction in the sand. If I drive on the road, the temp is fine in both stop and go and normal highway driving. I have put the air back into the tires. This is only happnig at the beach in 4 whel drive thru sand. Any suggestions? Thanks in advance. Bob