Just pulled the driveshaft to replace hanger bearing and couplers. I noticed the rust color before, but assumed it was just surface rust. Upon removal and inspection I saw it had apparent rust scaling and pitting. Would hate to see the condition of one from the rust belt. Anyway, it doesn't appear to be structurally compromised yet, so I took a wire brush and ground the rust away and put a few coats of primer/flat black paint on it. I don't recall ever having this problem with a drive shaft before, and being close to 50, I have owned many cars. Would it have killed Cadillac to put a coat of paint on???