Do you have access to a vacuum gauge? If so plug it into the vacuum line coming to the modulator and see what kind of vacuum reading you have at idle. You should have relatively high vacuum. If you don't, you likely have a broken vacuum line or a rotten vacuum connector somewhere. Lack of vacuum will cause a very hard shift.

If you have access to a hand vacuum pump, try pulling a vacuum on the modulator. If it fails to hold vacuum, the internal diaphragm has failed.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)