Towing typically unloads the front axle, so going into AWD seems pretty natural to me.
Not going into 9th doesn't surprise me. Towing a 2012 Rockwood Roo 19 (~4000 lbs) with my 2011, it would never go into 6th. Spent most of it's time in 4th, might go into 5th on a good decline. Any little increase in slope would drop it into 3rd. (I'm assuming your pulling something fairly heavy, i.e., >2000 lbs.)
Not sure what you mean by "rpm's drop." Relative to what? For the same speed, if you're not going into 9th, the rpm's should be going up.
2011 2LT FWD replaced by 2017 2LT AWD