It SHOULD shift softly with light throttle and harshly with wide open
The softness of the shift is dependent upon the amount of vacuum
supplied to the transmission's shift modulator. Lots of vacuum = soft
shifts and no vacuum = hard shifts.
Your car may have:
a broken or disconnected vacuum line to the shift modulator, or
incorrect vacuum being supplied to the transmission shift modulator, or
a broken transmission shift modulator.
Any of these is a relatively simple, and not expensive, repair. M-B
didn't sell these cars to shift harshly, even in the '80s.