Farmer Petry, according to the GM heritage site, a 2 speed axle was not an option for the 1-1/2 ton trucks. I was fortunate enough to find from one of our big bolt members a 2 speed pumpkin for my 2 ton axle. You could consider swapping in a complete axle from a heavier truck if you're planning on loading the truck heavy. Or, with the largest diameter drive tires you might be able to achieve your goal with the axle you have. My experience is this, my original gears were 7.20 :1, with the 2 speed pumpkin I now have 6.40/8.20 or there abouts. With 42 inch diameter drive tires my truck can hit 65mph, that is wound out against the governor at 4000 rpm.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy