What kind of differential do I need for towing?

Chevy Van G20, 305 engine, 3 speed transmission w/ overdrive. Does fine on flat trips but mountains it slows to 25 mph towing a pop-up camper. Springs handle the load just fine. Looked into differentials and saw positronic ratings from 28 to 33. What am I looking for? Higher or lower? What do these numbers mean?