My only guess is that the forward axle is getting more air flow. But that is really a weak guess.
Many truck weight stations now have infrared camera to take a picture of the trucks coming into the weight station. Any rims that are "Cold" indicate a brake not working. Any really high temperature reading indicates a brake that might be dragging.
Chances are that a dragging brake would be well over 200F, while one not working would have been closer to the ambient temperature.
My only question is how much did you use the brakes before measuring the temperature? It seems like the hubs are not much more than the outside air temp (you stated 87F). Normal running on a interstate would be about 10F - 25F warmer just because of friction in the hubs and tire flex. If you had been using the brakes a bunch, then they should have warmed the drums more - if the amperage was set high enough to have effective brake pressure.
You can also test each brake drum with a compos. Hold the compos near the drum, have someone push the brake on manually, and the needle should point N toward the rim.
Fred.
