Driving along in my 07 Classic Silverado felt a "clunk", truck "jerked" but continued on normally. Occasionally felt additional softer "clunks" and one quick growling sound. This is a LT 3500 with 177K, 4wd, dually. All axels, u joints, etc. seem tight. Running in 4 and high truck performed OK. I visualized a gear tooth breaking off in a gear box. Does this sound like a front differential issue???

 

It could be, but it also might be from your transmission, IMO.

 

I hate to think my Allison might be messing up. It would be nice to have metal chip detectors in each box to know which one might be coming apart. I could open a drain plug and let the lube drain out over a magnet...