If I understand correctly that wouldnt be the case. it is hyrdaulic pressure that holds it in 2wd, it is spring loaded to be in 4wd all the time. it's a clutch and the hydraulics are keeping the pedal pressed down so to speak.It may well be, I will remove the shaft at the weekend and give it another try. I did wonder if some dirt is stopping the solenoid closing completely, so when at high revs the higher pressure allows enough oil through to re engage the 4wd.
A lad called John Metcalf on one of the Ford groups on Facebook had a similar problem with his old 8210 whining on the road.Tried a different tactic at the weekend, driving down the road, turned the 4wd switch on, I get another high pitch noise when it's engaged, which stops the second I turn it off, so I don't think the 4wd is my issue. Also found when being pushed down hill the whine completely stops so it's only when it's actually pulling or under any load.