The Eaton HO33 axle wasn't offered with the Posi option until '60, but that 3rd member will swap right into the older axles. A '59 Posi will be the Spicer 45, same axle found under the GMC. 3.92 was a common GM ratio in the Spicer 45, as well as the Spicer 44 used as a front axle in the '60 and later 4x4. I don't beleive positrac was an option on 4x4 GM trucks until the mid-'70s.
A '58 with a Posi, especially if it can be confirmed as a factory install rather than a later dealer or aftermarket swap, would definetly be an odd one, as the option was not officially offered until '59 in trucks (passenger got it in '57).

Bill Burmeister