Rear end problems? yikes

[Brently]

I have a popping coming from the rear end - it is a posi rear end in my 72 swb - just bought the truck and it appears to be the one that came in it - it says this is what it is in the glove box lid - i haven't checked the gears - the popping only occurs when i turn - it is like the axle stops turning when i turn a curve - it pops loud and jumps - but again only when i turn - this confuses me - i know nothing about rear ends so can someone help please? Brent