Strange noise from rear end

I have a 89 Civic 4WD wagon w/manual trans. Lately I've noticed an odd "thud" sound sometimes when I take off from a stop. The noise seems to be coming from the rear end area. There seems to be some resistance when trying to start out, it's almost like the tires are glued to the road and when the car moves, they break away from the pavement, making that sound. Once I start moving, everything is normal. Does this sound familiar to anyone?
Comments
What are the effects of disabling the RWD by loosening that bolt and sliding that piece over, that under the hood, besides not being able to use the RWD feature? Would it hurt anything or cause any problems? Would it improve gas mileage?
Cheers
Cheers!