the rear end still kills it.
It certainly doesn't help. The purpose of a diesel IMO is that you don't need that low rear end to pull. 4.10's belong on gas burners. Diesels have gobs of low end torque and really don't care how much you "bog" them. Diesels don't need to rev like a gas burner. The duramax has a pretty high power curve for a diesel though which is why you get that neck snapping top end out of them.
People want their trucks to take off like corvettes these days though, so you'll find plenty of 3/4 ton diesels out there with 3.73's and even 4.10's in them when they should be wearing 3.42's.
Even still, you're not going to get the mileage you got in the old diesels, even with highway gears. They're EPA'd to death.