My Fiat 2.8 Ducato 14 has developed a worryng shudder at approx 40 MPH under light load.The whole cab shakes to and fro alarmly.
Once above this speed alls fine,changed fuel filter,checked engine mounts and wheel nuts.
Starts OK,revs up OK,no smoke from exhaust,no oil consumption,fuel consumption appears ok.
Possibly points to drive shaft ?

Hi, when you say 40 mph, what happends if you are in a different gear i.e. engine speed not road speed?

I would suspect broken steel brading in one of your tyres. you may not be able to see this yet. Can you feel anything in the steering? If not it could be rear tyre. Time to try your spare on each position.

Swapped the wheels around,fitted the fronts on the back and vice versa,shudder has reduced dramatically,so you could well be right in a faulty tyre.
A process of elimination then with removing one wheel at a time and replacing with spare(which has recently been shod with a new Barum).