I as well have been receiving the three messages on my '05 SE w/ HD package, starting with the HDC system unavailable, trannmission fault - stop safely, and then slightly afterward suspension fault and special programs not available, along with the accompanying warning lights (DSC, etc) and the lowered suspension (although sometimes it doesn't seem to go all the way to access height). No change in the suspension height indicator at the time, but after a restart, the HDC message appears briefly, and the suspension indicator shows offroad height while the truck remains at access, with all other faults gone. Selecting normal with one movement of the selector raises the truck back to normal (opposite to the movement of the selector) and all is good for between 2 and 5 kilometers, at which point it all goes again.
I have done some digging here and elsewhere, and beyond the additional details noted above, I am thinking 1) rear diff or 2) stuck parking switch. I had the same faults before with a weak battery, but can confirm the battery is still good and alternator seems strong (over 100 amps output, but gauge cuts off). I did replace the compressor several months ago with the updated version and haven't had it in for any software updates. Anyone else with additional specifications during/after the faults?