I had a 75 Duster that had the same problem and the alignment shop that did the work on the car the owner, who did most of the work had been in the business for over 40 years, told me that the box was a bad design and having a rebuilt put in would last a couple of years before it developed the same problem and sure enough it did. Had I kept the car I was going to change it over to rack and pinion after seeing an article in, I believe, Rod and Custom about a guy who dropped a Hemi in one and went that way because of clearance problems amongst other things.