So I'm cleaning up Sunshine Sidings, moving the dead ghouls out into the woods and I come across a dead legendary that's "this item is too heavy to move". Great, in the best house in the joint too.
Anyway I move on clearing other bodies and there's one who's head has been blown off (head's still there, just not attached). Straight away I note he's much lighter than the others, I can run with him. I go back for the head, pick it up and I'm thinking I might keep it as a trophy when I drop it (actually I think it more like jumped from my grasp) and it starts rolling very unnaturally and straight in the direction of where it's body is. It got caught on a stump or something but still wanted to move so I helped it and sure enough it keeps going (with a prod or two) until it finds it's body and finally it's at rest - it WANTED to be reunited.
Remembering how light the body was it gave me an idea. I went back to said "this item is too heavy to move" legendary ghoul, pull out the .44 and keep shooting till I blow his head off and you guessed it I could move it! And it's head wanted to be reunited too, which they are now dumped out in the bush and out of my settlement.
Hope it helps someone, if nothing else play reunite the head and see how far you can go getting it home on it's on steam.