1994 E320 Runs rough in gear.

[E320wagon'94]

My dads 1994 E320 started running really rough the other day. It smells really strongly and smokes. Under acceleration it sounds like its not firing properly. In Park it runs perfectly fine and revs smooth and freely. Once shifted into drive it runs rough and has little power. We replaced the O2 sensor and spark plugs today (plugs were 2000 Mi. old and very black.) I have no idea why it would do this, and only in gear? It seems to be running rich (not sure though.) any help would be greatly appreciated thanks.