Next time you have the sluggish problem, drive it a mile then stop and go check your rear brakes to see if they are warm... if they are then your e-brake is probably dragging, if not then there is another good possibility.
You say it only does this on really cold mornings; chances are good what you're experiencing is just the effects of temperature on your tranny and rear end oil along with the grease in your bearings... once they warm up from a little driving they thin out and things loosen up again.
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