1984 Dodge Truck differential

You already have a real low axle ratio. The overdrive is there to make up for it at highway speed. In the 1970s, the 4spd didn't have an overdrive gear. 4th gear was a 1:1 ratio. In your transmission, 3rd gear is the 1:1 ratio, so driving in 3rd gear is the same as driving in 4th gear in the 70s.

You can change the gear ratio but you're better of looking for a different axle because the ring and pinion set is expensive. Setup is critical too to prevent gear noise.

Caradiodoc