07-up with Nitro and Liberty, Liberty's were 3.7L V6 only, Nitro had 3.7L as base, 4.0LV6/5spd as optional. 3.7Ls come with the old 4spd auto only, as it sounds like you know.
My wife had a 2010 Liberty Ltd 4x4 that we bought new in Dec. 2009. 3.7L 4spd Selec-Trac 4x4, leather, heated seats, 18" chrome wheels, tow package. She loved it, I'm 6' tall and had VERY little leg room up front, she's 5'2" and was just fine. Liberty with the shorter wheelbase has less front leg room (by A LOT) and overall shorter front/back. Nitro is longer in front and back, Nitro offered the sliding tray out back that I don't think was available on Liberty. Liberty's use a toggle/switch for 4wd operation, while Nitros typically use a dial on the console.
Other than that, very similar vehicles with many similarities. Around here, many Libertys have the Selec-Trac fulltime 4wd (2WD, 4WD AUTO, 4WD LOCK), whereas every Nitro I've ever seen has the parttime 4wd (2H, 4H).
Ours got 17mpg combined, would get 20-21 highway if you drove sensibly. As our needs changed, she traded for a new 2012 Patriot Latitude in May, 2012, which gets MUCH better mileage and has MORE room inside!
Also, I do my own scheduled maintenance, and the oil filter location on the 3.7L in the Jeep Liberty is AWFUL!!! NOT FUN!!! I HATED changing the filter on that (at home, on the ground, no lift)
Good luck!
JS
