I have this problem as well. 2006 4.4L; 82,000 miles. Just bought it and did not notice it during the test drive or until a couple of weeks after I bought it.

I typically see it when I'm driving about 5-10 mph, coasting and then get back on the gas.
It is a fairly violent thump.

Transmission fluid is full.

I tried the 30 second gas pedal reset and it did not seem to make any difference. It also happens when it's cold (within 10 minutes of driving in the morning).

Not really thrilled here.