My exhaust "walks" while driving too...though doesn't move as far as you've described. Every day I notice my muffler has moved over towards the rear diff...and every day I move it back.
One quick check...did you put back all the exhaust hangers? Maybe the one on the midpipe isn't connected...there should be three total, one on the midpipe, two on the can.
The stock rubber hangers are VERY soft...too soft if you ask me. I'm going to invest (someday) in some stronger ones.
That's all I can think of... sorry!
NC