I don't know about the skid plate, but I can answer the rest of your question. I've got a 2013 Rubicon 2-dr with the 6-speed (0.797 top gear ratio + 4.10 axle), and stock tires (255/75R17, so they're 32" in diameter, or about 100" in circumference). The automatic's top gear ratio is 0.83, so pretty close. I hit 2000 RPM at/around 58 mph. I've got an hour long freeway commute to work (not for much longer, hopefully!), and cruising at 75 mph, it's around ~2600-2700 rpm (it's a little hard to tell exactly, especially since I can't maintain speed for long with the traffic). The gear ratios came directly from Chrysler, here: http://www.media.chrysler.com/dcxms/...nltd_Specs.pdf
(Transmissions for the 2013's are the same.)
The math seems to support our observations - at 70 mph, the tires should be turning at 739 RPM. (70 mi/hr * 5280 ft/mi * 12 in/ft / (60 min/hr) / (100 in/rotation) = 739 rotations/min). With your 0.83 top gear and 4.10 axle, your final drive is 3.40. That times 739 RPM is 2513 RPM, the engine speed you should be seeing at 70 mph. There's been a lot of rounding in this process, so I think your number is reasonable.
My assumption would be that the short axle is aimed at customers who want to upgrade tires without regearing.
FYI, I'm seeing right at 18 mpg during my 45-mile, 75 mph highway commute (with about 4 miles of stop and go traffic at ends).