2002 Checy TB 4 WD Problem

I've been told I need to have the front differential re-built or replaced with a factory rebuilt unit on a 2002 Chevy TrailBlazer. $2000-$2500. Was told that the problem was caused when I replaced only two tires when I had 2 flats from nails at a construction site. Mechanic said that computer senses the difference in the treadwear and was trying to compensate for that and it ruined the front diff. Does this sound legit? Someone said this is a common TB problem. I have 101,000 miles. Would you trust a auto salage yard unit with 28K on it or go for the factory rebuilt unit with a warranty?