talked to a friend today who's an encyclopedia of GM info - said the same as Bill, "posi" not available until '59, [but maybe the '58 was very late production and special order] ..... he also had a dash tag from one, which doesn't say positraction, but limited slip .... and he said the 3.92 Spicer 45 axle started on the GMC's about 56, so if anyone has GMC master parts books it might be found there
Bill
