My 86 K5 Blazer makes a popping noise when i accelerate/ brake. Has 6" lift & 35's. what could cause this?

I have a new tranny and new 4.56's front and rear. I installed the the tranny but had 4 wheel parts put in gears. It only pops once when accelerating/ braking hard. it almost sounds like my axle is shifting on the leafs. maybe a broken motor/ tranny mount?|||Your axle probably is shifting on the leaf pack. It is not uncommon for the center pin in the leaf pack to shear. The pin is what keeps your axle in place. Also, retorque the U-bolts