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Lack of power was caused by a piece of slag, yes a big honking piece of metal, in the turbo. Cured by installing a new turbo (with all the sensors!). My service tech said that Ford told him it couldn't be flushed out. It seems they were familiar enough with the problem that it must be fairly common. I found lots of rubber bands and tape pieces under the truck when new and wondered what sort of crap might be hidden- now I know. Also: replacing rear main seal does not require replacing oil.
I had read a while ago that this was an issue in early production. Something about slag coming off some of the welds in the manifolds, or something like that.