Update to my post above made on 05-17-18:
At the same time that they checked out the transmission, I had two new O.E.M rear leaf springs installed by them. Since then, I have a vibration that starts at about 40 MPH. I rotated my tires myself and while the rear tire was off I noticed that the axle is sitting back further being it was no longer underneath the rubber bumpers. I then checked my wheel base and noticed it's 171"! Since the tire rotation made no difference in the vibration, I called them and scheduled another appointment for them to check things over.
When I showed up for the appointment I explained my suspicion that the leafs were installed backwards, including showing them a picture of the axle's location in regards to the rubber bumper and mentioning the mysterious length increase . After a couple of hours, they reported back that the van does indeed have a vibration but they checked the torques and everything was tight and their conclusion was "tires". They had no explanation on why the w.b. was now 171", but did explain that it was "impossible" to install the leafs on backwards because the ends of the springs were different widths.
When I got home, I crawled underneath with my micrometer and measured the widths of the ends of the springs: approx. 2 23/32" EACH. No one end was wider than the other. I then measured from the center of the front mounting bolt to the axle: 27"; rear bolt to axle: 26 1/4". Looks like they did the impossible.
So now the van is headed up to Mechanicsburg, Pa., where a mechanic said that, "It's absolutely possible to install them oriented 180 deg. wrong". I'm keeping my fingers crossed that American Bus and Sprinter is open to reimbursing me for the cost of having someone else repair their screw up.