I don't know if your's had an earlier 3.4 vs the 3.9 in my '07. The 3.9 is a all new engine with variable valve timing. I think the only other pushrod engine with variable valve timing is the vette. Unloaded, mine gets about 22 cruising at 70+ while barely breaking 2000 rpm,. I can't explain your experience, perhaps the work vans have nasty shocks, tires, springs or something. Ours has been a very comfortable tow vehicle. Much better than any other I have had.

bob