Modern trucks should be doing fine with cruise control and tow/haul engaged whether going up or down a grade. On a RAM, it will try to maintain the selected speed and allow at most 5mph over when going down a grade before it gets really aggressive at slowing the vehicle. On my RAM, it starts with down shifting, then it starts using the exhaust brake if that isn't enough.
As someone else mentioned, GM and Ford both allow higher RPM when braking on their Diesels. Somewhere just a little north of 4,000 rpm.
OP seems like very odd circumstances to have your truck do what it did. It may just be a 1 in a million occurrence. Sounds like no harm has befallen you since you were able to drive just a bit later. If you damaged your engine, you would immediately hear some knocking or really bad ticking depending on whatever broke.
