Two Speed Rear Question?

I'm looking for a donor truck but I don't know what I'm looking for. I know I want a straight 6 and it has to be a Chevy in the late 50's or early 60's with a 2 speed rear. What chevy truck is it? I could give two ****s on the body, just looking as a donor of parts and pieces.

Please help.

 

Probably a 4000 or 40 series or larger truck, from the mid 50s to the mid 60s. Not all had the 2 speed rearend. Also the 2 speed just gives you a lower gear, not an overdrive....