popping in front end when in 4 wheel drive?
 

i have 99 dodge 4x4 cummins my cusin has same truck when we put our trucks in four hi or four low and drive the front end make a loud pop noise and shakes the the truck. feels and sounds like its coming from driver side. we have already check and replaced the vacume actuator on the front diff. the poping isnt constant it just does when ever seen to do it more at 20mph. its not u -joint or wheel bearing. does anyone have some helpful info?

That sounds like a u-joint to me. Did you check the drive shaft u-joints or just the axle joint?

axle u joints, ball joints

does it do it more when your turning?

tie rods can also do that when they go bad