I recall a friend had this vey same problem, as mentioned above critical to measure from chassis rail down to ground. After Many hours trying all sorts he placed a small jack under the front corner (same side as low rear) and gently jacked the car a fraction and the low rear corner raised! He could then corrected the the rear with a little lift at the front. It was eventually traced to a tight kingpin and bushes that he could only assume was 'holding' one front corner a fraction high and seemed to be pushing the rear down.
Might sound weird but that's what it was, he stripped the front, the pins looked fine so he cleaned and greased everything and the problem was gone.
You never know?
