An amature mechanic tells me that gearbox and clutch is fine but keeps pointing to the mounting ??? behind the engine and says that the shaking ? knocks the gears out and a round contraption needs changing.

Hi, you do not say what year the van is, or what mileage it is done, but your friend is probably on the right track with a worn mounting. l drive a Vauxhall Movano (Renault Master with Vauxhall badges!) and at 30,000 miles and one year old, it judders in first gear on pulling away, with a heavy load on the back.
Hope this helps!

engine mounting broken/worn