You should have an Eaton rear, (fully round housing) and if so you can get 4.10 ratio, lots of discussion lately about this swap just browse and or search a little.
I just made this swap I my 51 a couple weeks ago, real easy to do.
If you're concerned about traction you could purchase a Detroit Locker for a GM 14 bolt rear and it will fit inside the Eaton carrier. Then use 14 bolt axle shafts trimmed a 1/4" or there about. This would be a whole heck of a lot easier and cheaper than 4wd.
A 14 bolt axle swap is another option and they can be had with 3.73 gears and even some faster, as well as 4.10 and plenty of slower choices. Some searching will turn up discussion of this option too.
I too have only 2wd vehicles and rarely desire 4wd. If sometime I do really need 4wd I won't do that to one of my old trucks. Makes more sense to me to buy a ready to use newer 4wd vehicle.
