Billions of dollars worth of marketing has firmly lodged into the brain of many a vehicle buyer that they need All-Wheel-Drive or else they will instantly pirouette off the road to their death at the slightest hint of snow, or that they'll be relegated to pavement at all times and get stuck the instant they even look at a gravel road.
Reality is somewhat different. While AWD has it's uses, the capabilities are vastly over-blown and over-sold. AWD has very limited usefulness in rough or uneven terrain, and should not be relied upon to significantly extend the capabilities of your van into these areas. However AWD can mostly be left on at all times, and is very user-friendly and forgiving of mistakes.
For a great many people, especially people hoping to use their van off pavement, a RWD van with an aftermarket torsion-style limited slip or a fully locking rear differential will provide more capabilities when off pavement than the factory AWD system.
Neither are substitutes for an actual 4-wheel-drive system, which are offered by aftermarket upfitters.
Daily driving and helping to accelerate on short sections of slippery, flat surfaces. Think of moderately plowed snowy streets, or pavement covered in wet leaves, or well graded gravel roads with short sections of loose surface. The sort of conditions that you might reasonably expect daily driving in most situations.
It's also good for drivers who have limited mechanical interest don't want to worry about how it works. AWD is pretty forgiving, while mistakes in application of a 4WD system will lead to immediate and catastrophic failures (And not failures like “a warning light came on”, but failures like “shards of metal exploding out from under the van”).
Rocky, rutted, bumpy or highly uneven terrain where wheels may temporarily lose contact with the ground. Or extended sections of deep, loose materials such as deep snow, sand, mud or very lose gravel where maintaining forward momentum is critical. In these situations the factory AWD system will frequently shut down if driven in such conditions for more than 1-2 minutes.
For situations such as these, a RWD van with an aftermarket locking rear differential will almost always out-perform the factory AWD system. See this video for an example of just how effective a locking differential can be!
Locking differentials can typically be installed into most RWD vans for $2-$3,000.
A 4-wheel-drive system mechanically locks the front and rear axles together, and the left and right wheels together if engaged with differential locks. This provides smooth, consistent power to all wheels no matter what is going on with the other wheels. This makes it much easier to maintain control and critically momentum when in challenging conditions.
AWD systems such as used on the Transit and the Sprinter don't have mechanical locks between the axles and wheels. Instead they use limited slip differentials and computerized controls to detect when a wheel is spinning and doesn't have enough traction, and then apply the brake to that wheel which forces torque to be sent to the other wheels. This video by CampoVans is a good example of how the AWD system works. At various points you’ll see an off-the-ground wheel spin, then stop. Or a wheel on the ground spins and skids, then stops. That’s the computer applying the brake to just the spinning wheel, so the other wheel maintains traction. Also note that the van will back the power off without the driver changing their foot position on the accelerator, making the van lose momentum and being more likely to get stuck.
The result is that while 4WD vehicles seem to glide smoothly through lose terrain and up hills, AWD vans tend to “hop” and lose momentum as the computer can't react quickly enough to abrupt changes in traction. This video from Quigly (who makes true 4WD systems for vans) shows the difference.
4-wheel-drive conversions are offered by Quadvan or Quigley Vans. The Quigley 4WD system has been approved by Ford for the Transit, and if installed when new Ford will still honor the full factory warranty on all non-Quigley components of your van.
Expect to pay ~$15,000 to have a stock van converted to 4-wheel-drive.