Chris,
These things aren't rocket science, and are easy to pull to pieces, and rebuild, if there isn't any broken wires in the Armature or Field Coils.
The Commutator could need rubbing with a piece of Emery paper to clean it, the Brushes could need replacing, the Bushes could need replacing (No bearings in these Starters).
Plus, the Solenoid could just need the contacts cleaning or turning, and the Bendix could need replacing, if it is worn out in the roller clutch.
They are like all Basic GM Starter Motors, and the Cadillac uses the one with the heavier duty field coils.
You do have a Shop Manual? If you want to give me a ring, my number is in the Membership Directory.
Bruce.