Jeff,

BTDT - Seen a few of these before. Basically it's a rust issue because the rotor hub is so thin. GM had a recall on 88-92 1/2 & 3/4 ton 4x4 models and 90-92 all wheel drive Astro because of this (COMPOSITE BRAKE ROTOR CORROSION #92C34A - 10/01/1993) Login to read more.