2000 caravan transmission

2000 Caravan, 58000 miles, lately I have been having trouble I think it is the transmission. Sometimes when am stopped at a red light but mostly happens when running at different speeds, It feels like someone is lightly bumping me from behind. Think it might be transmission shifting, or trying to shift gears. Need to know what you think the problem is and the fix.
Thank You
Happy New Year
William

it sounds like a speed sensor problem. it wouldnt be a bad idea to ahve someone look at it because those do have problems with the valve bodies too.