Hello Owen,
That's a honky motor you have in your van if it's flexing the driveshaft! I've only got a 2.5 diesel in mine. Anyway, I had this problem albeit in a Peugeot car. I think your problem may be worn shoes binding against the drum even when you let off the handbrake. In some cases the shoes bind tighter as you try to drive away.
Try going backwards to free the shoes if indeed this is the problem. This might work. If it does remember to change the shoes ASAP as worn linings can drop off the backing.
I doubt you have a sudden problem with the differential. Your ears would have told you long ago a problem was going to occur !
Good luck, John
John Tucker in sunny Rye..., May 2007