From this web page: http://www.6066gmcguy.org/spicer-5831-b.htm
The 5831 three speed's were rated for 275 ft. lbs and 150 HP.
These were used in Chevy 5500 series trucks in the '60's.
I don't know what type of payload they were rated for, I'm thinking it would be hard to overload one in a typical pick-up.
I know heavier duty four speed's were built as well.
Try a search on Spicer auxiliary transmission.