So, I'm pretty sure I've resolved this. The problem, as far as my machine goes, was due to A2A's accufeel and Realview from Realair. I had the A2A commanche installed, as well as the RealAir turbine duke. It seems that RealView affects all aircraft, not just the duke. Turning all the effects in the RealAir control panel to "off" seems to have fixed it. I haven't tested it completely, but I think this is it. Got the idea to check this out via this thread: http://www.avsim.com/topic/479433-accu-feel-v2-for-prepar3d-v3/page-2
edit: Nevermind. Its still leaning. However, changing all the realism settings in P3D all the way to the left to "easy" reduces the problem. So it might still be related to accufeel or something similar.