Usually worn/cracked bushings on the shift linkage will cause the problem you describe. Thomas Pendelski makes it look easy on his web site. Some people even replace the bushings without removing the linkage from the car, but pressing them on with a socket and vice-grips as a "press" takes a little finagling.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)