In addition to psfred's good comments, I would add:
If the rear end tends to feel a little loose at highway speeds, like its being blown side to side by the wind .. check the rear suspension-
The rear suspension has five links per side. The upper diagonal link can contribute to the symptoms you described. But other worn parts can also contribute, some more expensive while others not very hard to replace or expensive.
The rear sway bar bushings are cheap and easy to replace as are the vertical links connecting the sway bar to the rear suspension. On a 1991 the bushings might be worn check the links.
Further, do the front sway bar bushings as well. Again easy and inexpensive and the results are great! (No link on the front) This is something a non-mechanical person could do. I did this on all my cars.
91 300E's, great cars!
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)