Nothing needed from the university offices except a copy of the tenure-track job offer letter (this shows that your job is permenent) and a support (or cover) letter from an officer who will sign I-140 (your chair is OK).
All other supporting materials on research/teaching are from yourself.
Trust me.
I did this myself. Number of reference letters = 0 . Believe it or not.
Approved within 4 days on March 2007.