Bed mounts ('98 F-150) have rusted through and now the bed moves slightly behind the cab creating a bad noise. Does anyone make replacement mounts that can be welded to the frame? Or is there any other repair/replacement solution?

 

Perhaps you can find some comperable sized boxed tubing and make new ones?

 

That is a lot of rust. Removing the bed isn't that hard. I would pull it off and work on it to secure it. While it's off you can inspect the rest of it for issues.