I've got what sounds like the same issue. I can't really recreate it by shifting from drive to reverse and back again as you would be able to with loose suspension or u-joints. It's my wife's daily so I don't drive it enough to really figure out the repeatability. But its noticable for sure.
I want to make sure I read the instructions correctly. Essentially you should have roughly 3mm of window in the weather stripping when the door is closed?
Don't believe for a second a dealer that has a Bronco sitting on their lot is going to tell you the engine has been replaced or not. It's not dishonest to sell something that's in perfect working order, even if parts have been replaced to make it in perfect working order.
In the 21211 gang, 6100 miles.
I'm convinced certain days got certain heads with good/normal valves. Other days, not so lucky...21224 is a bad day. It's still possible some heads on 21224 we're good, just not all.