I had this issue with my Nvidia GTX 570 and Radeon 6870, and my cousin also had it with his Radeon 5830. I was messing around with different .dll files one day for performance imrpovements on my GTX 570 computer, and noticed the trees had started swaying, so I went and checked what was different from the GTX 570 machine and my 6870 machine. I noticed in the Skyrimprefs.ini file,
fTreesMidLODSwitchDist was set to 0 by default(even when I had selected ultra), but one of the .dlls made a config that had set it to like 4000 or something. Right now I have fTreesMidLODSwitchDist=12000 which is a good spot for my system. The higher the value, the further away the trees sway. Try it, hopefully it fixes your issue
