"This winter I had my steering gear box and the coupler rebuilt"

Did they rebuild the box, or replace the box? How about the coupler?

I'm thinking one or the other was replaced and now its not fitting properly on the splined input shaft, basically literally allowing the connection to 'slip'.

I had this happen on a Dodge truck I used to own. I replaced the steering box with a rebuilt unit, put everything back together and made sure everything was tight. A few weeks later, I caught a puddle with the right front and steered left to compensate. After I hit the puddle, the steering wheel was no longer centered.