Transmission shakes above 50mph mostly upon acceleration.
When was the last time the transmission fluid was serviced?
How does the OP know that the shaking is coming from the transmission?
well. if you took it to aamco they would say you need a new trans. take it to dealer and they would say you need something real expensive and try to sell you a new van. a shop might say its motor related. hard to say
Try a few experiments. Start coasting in D at 60, coast through 40. Next try that in N. Next try it in D with brakes slightly applied. Then in N w/brakes. Next accelerate from 40 to 50 in D. Try it again in the next lowest gear. Note what happens in those situations and report the results here.