Well it is an F550, so its really not meant to be going 70 all day getting optimal MPG. 4.11's or 4.30's are a good drop down which would bring the RPMs back down a bit.
How big are your tires? If they are stock 31.4" tires, from my calculations you should be right at 80mph at 3000rpms. Gearing to a 4.11 would put it at 96mph at 3000rpms.
Not sure on the 2 speed rear, but i believe it is a dana 135 rear. Here is some info i just copy/pasted from another site:
There are 17 different ratios available for the Dana S135-S axle, including both the 3.91 and the 4.10 ratios that texas550psd asked about.
"Here are the Dana/Spicer part numbers for the matched ring and pinion sets on the ratios requested:
3.91 Ratio Ring & Pinion Set: 360KG105-X
4.10 Ratio Ring & Pinion Set: 360KG106-X
An important HOWEVER:
Since both the 3.91 and the 4.10 ratios fall outside of the 4.88 - 5.38 range of ratios the stock differential case uses in both diesel and gas configurations, a different differential case will also be required to use the taller gears. (By "case" I do not mean the external housing, but the internal differential case, ring gear backing plate, and bolt group.)
The Dana/Spicer part number for the differential case group appropriate for the range of axle ratios from 3.91 to 4.44 is:
Spicer Heavy Axle and Brake Division
Fort Wayne, Indianna"