Dylan's strumming patterns are strange, and can be very hard to replicate, partly because of his love of irregularity. You can rarely keep a precise pattern throughout if you want to mimic exactly what he does. However, there are some basic patterns that uses, especially for Blood on the Tracks, where he seems to be using a particular style.
Here's an attempt at showing how he strums.
The song is at a tempo of about 108.
If you tap your foot at that tempo, then do this:
Play downstrokes on quaver beats and upstrokes at the semiquaver beats, if you get my meaning; ie, count every quaver as a downbeat and every odd semiquaver as an upbeat.
Strum two downstrokes to the first beat at quaver intervals. The second downbeat should last for a dotted quaver though, taking you a semiquaver in to the next beat. Immediately play an upstroke on the odd semiquaver then down on the quaver then up again on the last semiquaver before the third beat. Repeat the pattern, which lasts for two beats.
Hope this helps. If you don't uderstand what I'm talking about please tell me and I will try to correct it.
By the way, thanks for asking the question, it's made me get Blood on the Tracks out again, which I haven't listened to in a while - great album.