Why does the 4x4 click a time or two when I first take off pushing snow

What is your question?

I have a click or two when in 4x4. 2000 silverado 2500 hd manual shift transfer with auto transmission. This happens, not every time) only when I start to push snow. New CV joints, hubs brakes calipers rotors lines etc, the front driveshaft is tight as well as the u joints. Was going to replace joints but the rust makes that almost impossible. Also the front wheel jumps a little when it makes this noise. Is it possibly the chain or the u joints that's my question. The transmission has been replaced with a Jasper, before I bought this truck, nothing seems to be leaking more than a little fluid, and there is no slack in the front differential, or I joints but there is a little slack at the transfer case output shaft where it connects to the front driveshaft, and the slack is not in that u joint

What makes this problem better or worse?

Only when I start to plow snow

How long has your 2000 Chevrolet K2500 had this problem?

Just happened within the last 20 miles