I've got an automatic w/4.10 gears, but 265/70-16 (~31")tires, and my truck turns about 2100rpm at 70mph, and idles (fully hot) at around 600-650 with the A/C off.

You've got taller gears, but smaller tires. I couldn't find the specs on the differences between the 5th gear ratio in your tranny and the O/D ratio in my automatic, but the rear-end gears versus tire size should make your driveshaft turn about 6% slower than mine at the same road speed. Again, the top gear ratio differences between the tranmissions could change your actual difference in crankshaft speed up or down from this 6%.

Another possible difference is in our speedometers -- I've checked mine with a GPS and it's pretty close. At 70, a 4mph difference in speedo's could erase the tire/gear differences above.

That said my guess is your tach is at least reasonable.