OK, got some good close-up pictures of how this park lock is supposed to work.
Here's a shot of the lock plate and the tab at the end of the shift lever:
However, there seems to be a problem. It seems that the tab at the end of the shift lever doesn't extend far enough into the slot to truly lock it into place. In this picture, I'm holding the lock plate down onto the end of the shift lever, and it's as far as it can go:
As you can see, the tab doesn't even extend halfway into the slot!
If the tab at the end of your shift lever is worn at all, then it's going to pop out pretty easily. Mine doesn't appear to be as bad as yours, but it still seems to be worn enough that it wouldn't do you much good.
Sorry I couldn't help you, but at least we now know why these trucks pop out of gear so darned easily! Man, this is a poor design.