I have dealt with this on 4 wheel drives that had otherwise good suspension- bushings, tie rods, etc. The oscilating you describe, particularly after hitting a pot hole on one side is exactly what one of my trucks would do and you are correct- there is no driving through it.

Check the tightness of the wheel bearings- I know it sounds different or weird but I had just rebuilt my front end, just like you, and sometimes with all that fresh grease etc., you need to go back and retighten. I have a tendency from my drag racing days to set stuff up too loose.

I did that and the truck was perfect.