Fronts Superduty axles should be 35 spline.
Your rear axle, if it's original, should be a 35-spline semi-float Dana 60.
Any idea how heavy your van is on the rear axle? Heavy extended-body vans with a heavy buildout, often exceed what that axle was designed for, and there's been several failures here over the years. If you're over the stock rating (6,350 IIRC) or you're running much larger than stock tires, you might want to sink you $ into a stronger axle instead.
For what you'll spend on a locker, you can probably pick up a used F250 axle instead, and those have a factory e-locker option, and will match your front bolt pattern.
2000 E450 dually V10 wagon