I have a 77 k5 with a np203 it makes a loud bang about ever 30 ft when in 4 hi. It does have front lockouts.

 

where does the bang come from?

have you checked the gears in the truck to make sure they are matching because if it has lockouts the axle is most likely out of something else. there shouldnt be any lockouts on a truck with a 203. try running it in hi locked with the lockouts open. and see if you still get the sound

 

I changed the lockouts and no longer have the problem.