when starting off why does my gmc 1500 truck feel's like rear brake's are holdin

If it seems like the rear brakes are holding but the wheels spin freely -- I would ask, is it sluggish acceleration? Lack of power? Obviously the brakes are not holding the wheels back if the wheels spin easily.

when the truck is running and it's put into gear it's sluggish and doesn't want to go. it's like the brake's is holding it from going. but it's not the brake's, could it possibly be the transmission ??? i'm grasping for idea's.