I just saw something in which the medical examiner found no other trauma, which would be inconsistent with dying from actions inside the van, it seems. If he was thrashing around so violently as to sever his spinal cord, then it would stand to reason he would have bruising elsewhere. Shoulder, elbow, face, etc.
Then there's this:
http://touch.baltimoresun.com/#section/-1/article/p2p-83373499/
Which indicates that it isn't unprecedented for police to drive iradically with handcuffed people in the van so that they get injured. There were apparently other cases of them doing this and people getting paralyzed and killed.
But, again, if that was the case you'd think there'd be bruising elsewhere.