Also posted my follow-up on other sites;

Just for giggles--- In desperation I called the local GM dealership and spoke to their parts guy. Gave him the info posted above... told him it was a 12 bolt w/six lug axles.Told him I thought 70's era.
He said he didn't have a book listing those numbers but could call GM and would get back to me.
Called me back about thirty minutes later and said " your differential is a 10.5" ring gear 3.73 ratio out of an 81-90 one ton truck or an 81-93 panel"
Ok I'm not real bright but there is NO WAY this thing has a 10.5" ring gear.
I am going to pull the cover and try to get the numbers off the ring gear.
Anyone have or have access to a Hollander interchange book????
Thanks!!!!!!!!!