the only problem that I have noticed is that it seems like it shimmies in the back (in little wind) and sometimes no wind, and it also takes the bumps pretty hard ............ Does the van shimmeing and taking bumps hard have anything to do with the wheel base being further under the van or could this be a mechanical problem.
Short answer - yes it could be a mechanical problem.
Long answer: Having a vehicle with a longer wheelbase (distance between front and rear axles or wheels) should have absolutely nothing to do with what you are describing (the shimmying or harsh ride) - a vehicle with a longer wheelbase will ride smoother than one with a shorter wheelbase, all else being equal (same springs, shocks, etc.) Take a ride in a Jeep CJ7 or CJ5 and you will see
The shimmying could be anything from a tire out of balance, to a bad carrier (driveshaft support) bearing, problems in the differential (rearend), loose suspension components and possibly a whole host of other things.
As for the harsh ride, this is likely due to one or more of several things: over-inflated tires, bad or poor quality shock absorbers (historically Detroit has been notorious for putting poor quality shocks on as original equipment. On the otherhand my wife's 2000 VW Passat still has the original struts ... with almost 170K miles) and finally the springs.
Leaf or coil springs should be matched to the vehicle they are on - and the expected load they are going to carry - put on too heavy a spring and the vehicle will ride harsh - and possibly ride better when loaded with a significant amount of weight. Had friend with a Chev or Ford cube van - this thing had rear springs on it that probably should have been on a semi-tractor - rode like a hard tail Harley. Way overkill and gave a horrific ride
Ideally, the springs would be a "progressive-rate" spring - this allows more flex when the vehicle is unloaded and more stiffness (to compensate for the additional load) when loaded. The Sprinter comes with progressive rate springs from the factory, although it is somewhat harsh unloaded. (Stock shocks and struts seem to be el-cheapos though)
Another way to handle the quality of ride issue for a leaf spring vehicle is to put on "overload" or "helper" springs. These are an additional leaf that only becomes "active" when the remainder of the spring pack is deflected (bent) far enough when under load to allow it to contact the "overload" or "helper" spring. For vehicles with a coil spring suspension Moog makes a coil spring called "Cargo Coils" that is progessively wound - providing additional stiffness when a load is placed on them - but a more normal ride when unloaded.
Additionally springs do wear out and will lose some of their flex over time, and become collapsed. A spring shop (Stuver Spring in Akron) can re-arc the individual leafs in a leaf spring pack - although I have never been a big fan of this because I think once the metal has been fatigued enough to require re-arc'ing that the subsequent service life of the re-arc'ed spring is less than new metal - but it's sure better than running on collapsed springs.
You would be well- advised to have the shimmying checked out - it is a symptom of a problem - one that could be significantly more expensive to fix if neglected.
