I have the same problem in a serivce van with the same transmission.

We did a complete oil change, torque converter and pan with Red Line ATF and the prolem went away. You must drain the torque converter, there is a drain plug you can get to through the inspection window on the bottom of the bell housing.
The van started doing this at 100k miles, and stopped with the change.
After about 20 thousand miles the chatter will come back and we change the oil and it goes away again. We have been going like this for 120 thousand miles.(van and original trans have 220k miles)
Oil changes are expensive: 12 qts @ 6-7 bucks a quart.
Good luck it worked for me.
George