About 5 years ago my 84 300 d (Turbo) had difficulty shifting into 4th. I did not remember experiencing the hard shift to 2nd. For diagnostic purposes perhaps you could try running it up in 3rd on the highway then let off the accelerator, drop it into nuetral and then back into drive. My car would shift into 4th if I did this. If the proposed purchase car does the same, we may be looking at the same problem. Mine was diagnosed as corrosion in the bowden (sp?) cable. The mechanic greased the cable ($60) and it has been fine since. You may be able to get away with greasing or in may have to be replaced. Please note mine is a northern rust belt car where corrosion is prevalent, so yours may have a whole different problem.
Schooner