Check the online ETK for suspension setup on your car. Judging by the pics you have std 540 setup? If you feel the rear has sagged there are are several causes:
1. Shocks (when really old (but still actually damping) my Boges dropped the height of the car (front and rear) by 2"). This is no exaggeration as on replacement with the same part the car was raised that much...
2. Check for worn dogbones, trailing arm bushings and shock mounts. (But these make much less (or negligable) difference.
To get an idea of how much it is out, get the car on a four wheel aligner, but not for anb alignment (all they can adjust on our cars are the tie-rods (for toe). Once on the aligner have a look at the aligner's rear readings, and compare to the Bently's suspension settings page. Rear camber should be within limits, but if your car has dropped at the rear it won't be! If so- try raising the car a little bit (by taking the weight of the car with a jack) and push the diff up an inch, then maybe two. If the settings come into spec, it is probably OK but the shocks (most likely) have lost it and perhaps dropped the car. If it won't come on spec, there'll be some dead bushes in those trailing arms... I guess. Perhaps someone else can comment on what else it could be?
Rear settings:
Stock:
Toe angle 0 25' +/- 7'
Axis deviation 0 +/- 15'
Camber -2 20' +/- 30'
Sport suspension:
Toe angle 0 28' +/- 7'
Axis deviation 0 +/- 15'
Camber -2 45' +/- 30'
M-Technic suspension:
Toe angle 0 21' +/- 7'
Axis deviation 0 +/- 15'
Camber -2 50' +/- 30'
btw; There is only one adjustment possible without changing parts for specialty items- and it has nothing to do w ride height (ie on our cars the only adjustment we can do is turn the front tie-rods to get the front wheels within an 0.18'.0" +/-5' total toe angle).