Randy's Ring and Pinion has a nice rpm calculator on it.

I have 31.5" tires with 3.42 gears. I run 1500rpm @ 60mph and a hair under 2000 @ 80 mph. It just isn't enough gear for the 6.0. It's a dog down low and on the highway there is no throttle response. It was much better with the stock 30" tires. So I'm switching to AAM 4.10s. That should really wake up the LQ9.

 

I have the same truck with 265/75/16s on it and it is just something you have to accept. Drive your truck for awhile and then drive a truck with a 60, the 60 will feel like it is slipping because of how it shifts compaired to how the 80 shifts. Mine seems to turn about the same RPMs as you have stated, mine might be a little lower but not by much. I bet the person you bought the truck from, if they installed a SC, lift, and bigger tires. They probably changed the gears to a 4.56 or something

 

I'm running 33's myself and turn about 2300rpm at 70 with 4:10's and a nv4500.