Generally the wheelspeed sensors aren't great places to pull speed from - the output is very low voltage, very fast, has to travel a hellishly long distance in the car, usually along with high-amp wires (battery, fuel pump, etc), and needs some filtering to be accurate or consistent. The Dakota Digital boxes should have settings to bring the speed down enough, but I don't know if the output will be what you're used to seeing on a speedo.
My recommendation would be a mag sensor that reads the driveshaft nuts where they enter the diff - high-res, better speed (~16 pulses per wheel rev, vs 40), and less noise.