I love the subtle delay on Knockin' on Heaven's Door.
Jerry DID use a MXR Analog Delay. Back then they were larger and rectangle size. Today you can get the same pedal, now in the same size as any basic MXR pedal. The old pedals had the knobs straight across. With the new ones it's in a triangle shape. On the the new ones if you turn the Mix in the 12 to 3 o'clock position and the delay knob facing the mix knob up top you get a slapping/bouncing sound. I don't know if jerry did this but it sounds like he did to me. Jerry uses the MXR I THINK in Scarlet Begonias from the 1983/10/14 Hartford show, and he with out a doubt straight up uses it during Dancin' in the Streets from the 1985/11/01 Richmond show towards the end solo with distortion.
Here's the one Jerry had http://www.youtube.com/watch?v=9BLe2oTXwyk
Today's version http://www.youtube.com/watch?v=xJPde-C28iU