2007 GMC Sierra 1500 bang on backup

This truck has 226,000 miles on it Apx.(I bought it with 222,00)
Twice while starting to back in two wheel drive I would back a few feet then there was a loud bang like I hit something, ( there was nothing behind me to hit) it stopped the truck dead and made a ratcheting sound, truck did not move. I put in in drive, It worked fine.Today A week latter did the same thing. The bang was loud enough for people 50 Ft away asked me what was that noise. Any Ideas?

The transfer case chain may be stretched and catching inside the case as you back up. Same for something inside the transmission OR inside the rear axle housing around the differential.

That is as close as I can get over the internet, sorry.