I would say you won't lose your highway function, just need to slow down some. In our 4.56'ed Power Wagon, I have found that 65 mph seems to be it's happy spot, empty or towing, it keeps rear diff at around 175*F. The Hemi & G56 will certainly do much more, but the rear diff heats up a LOT more than @ 65. Most people don't give diff temps a second thought. I have got temps of 215*f+. empty, running @ 80 mph. Highest temp was 315*F when towing our 10,500 lb toyhauler on a 95*F day. I use an infrared thermometer & have verified it's reading with 2 other thermometers. Always within a few degrees of each other. I also use full synthetic fluid, always.
