"Thrashing," or "grating" Noise in Auto4WD

[Chubbs878]

I bought a 2003 Yukon 4x4 with 125,000 miles about 2 months ago. The "service 4wd" message in d.i.c. and seller repaired as purchase agreement; it would not change modes. Changed modes no problem when I got it home so I decided to test the 4WD. Got it on a muddy trail, trans in "neutral," selected Auto4 and slowly accelerated in a straight path. After 100 yards I hear a THRASHING sound coming from the front, steady noise, my momentum slows, truck pulls to the left. I stopped, selected 2WD, waited for light to stop blinking and slowly accelerated. The noise and driveline resistance ceased to exist. I have not attempted to change modes since then as I daily drive this truck. I would like to know what was hanging up in the driveline in Auto4 so that I can replace the parts and have a properly functioning 4WD system. I have never experienced any problems using 2WD, commuting about 20-miles daily, both city & highway environments. I have drained & filled both differentials with GM spec, Tcase with Delco AT2, trans' with Delco 6syn, no leaks or drips, checked hubs/wheel bearings: tight&solid, no issue with brakes. What happened in Auto4? Never attempted to drive in 4H or 4L modes.

[Heavy 5.7]

Probably not going to repair this over the interwebz, take it to a competent mechanic.