Ford 2001 E-150 Van Trans knocking 30 to 45mph

Passanger van, 4.6L with 104,000 miles Between 30 and 45 mph it will jump a bit. Best decribed as the feeling when the engin is knocking but the sound is not as loud. Had two garages check the motor and it runs fine. Aftre service it went away for awhile then returned. They said it is the trans because of the speed. Suggested a cleaning service that attach hose to filler tube to pull and replace fluid.
Can I just add a can of motor flush cleaner, drive for a few miles then change filter and oil?
How do I empty the tourq converter or will it just drain with the pan off?