The center differential is not a limited slip in the same sense as the Traction Lock rear end that uses forces from the side gears to engage the clutches. The center diff uses an electromagnetically engaged clutch pack that's activated by the computer once it decides from the readings it gets from the various speed sensors in the transfer case that one end is losing traction. When it does this, the front and rear are equally locked together. But when it is not locked, it does behave as a completely open diff.

As someone mentioned, you can jack up all 4 wheels, you can see if the system is working. With the engine idling and brakes on, put the transmission in any gear, and release the brakes. At least one front and both rear wheels should turn; I think all E4WD vans have LSDs in the rear. You can carefully grab one of the front wheels to try to stop it, and the other should start spinning twice as fast as before; the front diff is completely open. If you disconnect the computer under the driver's seat, you can actually try to stop both front wheels, and all power will go to the rear.