You presently have a 4.57 or optionally a 5.14 rearend ratio in a 3/4 ton.
The kook has a good writeup on a swap that will get you a little more speed.
If you can presently do 50mph with a 4.57 (which sounds about right) you will be able to do 55mph with a 4.11 running the same engine revs or if you buy a 3.90 kit (I believe sold by Randy's Ring and Pinion) you will be able to do 58 mph.
