Ron .... thanks for the the heads-up on the external/remote transmission filter!
I crawled down and looked - my 2005 E450 chassis indeed does have one - and it's been changed. There are fairly fresh wrench marks on my external filter canister's bolt head and I now remember a recent service where they mentioned changing this filter.
This filter should be changed at the regular 5R110W (5-speed TorqShift transmission) fluid replacement points.
Read all about it here:
https://www.ford-trucks.com/forums/search.php?do=showgcs&query=5R110+filter