1972 Dart Rear End Gear???

Its an 8 1/4. Mostly cruising in town but some highway cruising. I dont want the rpm's through the roof when cruising around 70 though. Right now it has a sure grip 8 3/4 with 2.88 gears. I'm not really sure what the sure grip actully does. I talked to a few people and they said it locks the rear end like posi. They said driving on the road with that wouldn't be good because when I go around a corner the tires would chirp.