Like CF said, there is no change to the high range. The gears only effect the low range.
With the dual case you have many gearing options. When in 2wd on the street, the front case is in high, stick pulled back, and the rear case is in 2 high. When you shift the rear case into 4wd high, that's all you've done is send power to the front. From there you can decide how low you want to go. Shift the rear case into low, then shift the front case in too if you want lower.
If you go with 4.70's in the rear, then you can put the rear case in 4 high and the front case in low for stock low range or just use the rear case for 4.70 or both for get out take a leak and pop open a fresh beer while you wait for your rig to catch up.
The dual case with 4.70's in the rear and stock up front is what Marlin calls his "dual ultimate" because it has so many gear choices.
Anyway you go, you'll be happy!!!
