What size truck do you have?
A 3/4 or 1 ton have interchangable differential parts from 1946-1972.
In the late fifties, sixties, and seventies, they made lower ratio (taller) gears in the 3/4 tons. You can get 3.73:1 and 4.10:1 off of those trucks that were originally equipped with automatics.
Usually.
If you're a half ton, check with the various vendors and Patrick's to see if the 3.55:1 gears are still available.
I would go with a gearing swap over an axle swap because the brakes that came with your truck are going to be better than the ones on a car axle.
