Here's how I've always 'slipped' in Logic:
First I double click the region, so the Sample Editor opens beneath it, then (inside the Sample Editor):
•Press Command, and use the mouse to move the selection left or right (in the sample editor)
•Press the key command for Selection->Region.
Compensate Region Position is OFF when I do this, if not, I get 'slide' instead of 'slip'.
A way to do this IN REAL TIME without opening the sample editor and needing that extra key command would of course more user friendly. Are you saying that this is possible in Shuffle Mode?