strange vibration?


When I come out of a corner hard, as in push through a corner and then lay down to full throttle exiting I get a strange vibration from the rear end. As long as I hold the throttle I will keep getting the vibration, but if I lift for a second then get back on the throttle things will be smooth again. Far as I can tell the vibration frequency increases with speed. I also never have issues when going full throttle in a straight line.
Could this be a differential fluid issue, or possibly clutch packs slipping? I don't see any indications that TCS or AH are kicking in when I feel this. I possibly added too much friction modifier last time I swapped the fluid, so recently I drained out a quart and replaced it with straight fluid - that seemed to help the issue (or it could be in my head) but it's still not gone.
Any ideas???

