I used to pull our popup with a Chrysler Town & Country, one that didn't have a tow package and felt the same sensation! Looking at the 2008 Ford Towing Capacity webpage, I would say that you may coming up to the limits of your vehicle's towing capacity. While you will get some see-sawing, it shouldn't be excessive if it's all loaded correctly. I'm not pro, it took me a few trips to figure out the best way to load both the vehicle and the trailer.
Of course, after a summer with the van, I went out and got a new F150 Supercrew 5.4 v8 with max tow package - that seemed to do the trick for me :-)
2010 Jayco 26BH
2010 Ford F150 XLT 5.4L 3.73 4x4 SuperCrew with Max Tow Package