Just to throw a wrench in the works, I have fixed numerous problem cars (cars aligned and worked on numerous places), that had their steering boxes tightened to tight. It causes a wandering sensation as you are constantly correcting that which the car should do itself. Improperly installed ball joints will do similar.
I mention this because you said the box was adjusted. It is very hard to adjust a box on the car and it should only be done by someone with gobs of experience. If you hadn't mentioned that I would have went with all the other good input about the rear links.
If you have the box problem it can get by a lot of people. If you mention the possibility it should be easy for them to verify as they do the alignment.
Bosch Master, ASE Master, L1
33 years MB technician