The diameter of your tires will affect your RPM's as well as gear ratio. In my experience the 3.90 gears are the most common in the 55T differentials. That rpm sounds about right for 55-6-mph. In my Suburban I'm running 3.42 gears with 275/60R-15 tires and at 70mph the engine it turning about 3200. Here is one of the online gear/rpm calculators. Thisone is for 4x4's and can be a little confusing. I know you can find others if you search.
http://www.grimmjeeper.com/gears.html