engine vibration at low speeds

What is your question?

I have a 2004 Honda Accord and it has been an excellent car. It has 220,000 miles because I put alot of highway miles on the car. I lost my job about a year and a half ago and now deliver newspapers at night to make ends meet and i do alot of stop and go driving. When my car is shifting into drive from 3rd(automatic trans), there is a vibration in the engine. Not a transmission slip but kind of a engine shake. It doesn't happen unless i'm going slow, around 20-30mph, like i do alot now stopping to drop off papers. I have added some trans fluid but have never changed it. It one thing i haven't done since it was new( I do all the maintenance myself, change oil every 3000, brakes rotors ect but its never been in the shop fro anything. Just curious what to look for with the motor shake or vibration?