New problem: Rebuilt the gearing tonight, pulled all the grease that had been packed in there from the previous owner.
Now the car sounds like it has a slipper clutch installed (like when you set an electric drill to a low setting and try driving a screw) but only when driving in a straight line. If I drive with even a slight turn, everything is fine.
My only assumption is that the diff I put in is stripped. Would this be a symptom of a stripped diff?