Rear wheel slipping out under load
I had a similar problem on my Pugsley, though it was under braking (discs) that I was kicking the wheel around in the dropouts.

After talking with the guys at my LBS, I learned that I wasn't putting enough force into the quick release.  They said that a good, tight, clamp should leave an imprint of the lever on your palm when you close it.  It will be difficult to open, but not horrible.  This measure of tightness has served me well so far.

I would also check to ensure that your clamping surfaces are clean (get rid of any excess lube which may have seeped in there).  You can also look at the skewer itself to ensure it is a high quality skewer.  The type of clamp (I don't know if the older style even exists anymore, but I do run into them from time to time) can make a difference as well as the roughness of the clamping surface.  For a while I was using a pair of Surly Tuggnuts which have a smooth surface facing the frame which made my problem even worse.