As far as I can tell from looking at my Hollander, they did offer posi in 58-59 1/2 ton Chev and GMC. Probably hard to come by these days. Not sure if anyone makes an aftermarket posi carrier for the 55-59 rearend.
A decent set of tires and a couple bags of sand in the back for weight will probably do more for you than posi, in the snow.
