What 3B and truckernix said... I have heard also.
I got a 4.10 from a 1971 3/4 ton with V8 and auto trans. It is an easy replacement, easier to replace than the gears in a non full float rear end. Lots of people have done this.
Beware, you can get a 4.57 gear that was also in the 3/4 tons and that won't improve your highway speed that much. They also have 5.14 gears in the 1 tons that you DON'T want unless you want to go slower. I've heard 3.90's exist, but never seen one.
Some of these differentials had the "no slip" option. This is essentially a detroit locker inside the carrier. It's a bit more rare, and might be just what you want. Mine (with locker) has a metal tag on the front top of the pinon support.
Only way to know for sure the gearing is to take it apart and count the teeth. Other than that, lift one side and rotate the wheel untill the pinion makes one full turn. Multiply the turns of the wheel by 2 and that's your ratio. If you can't get the wheel to rotate freely, then it's either a locked differential or a busted one.
Good luck!
