I don't think there's a limited slip (positraction) carrier assembly available for the 1956 OEM truck rear end. Some of the later-model trucks had 6 lug wheels and posi rear ends, but that would require an axle swap. Try to find a GMC Dana/Spicer rear end- - - -lots of aftermarket parts and gear ratios available for that axle.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!