FWIW the E-Series rear axle width resulted from Ford's reluctance to change so much of its production tooling when the 1992 models were being built. Factory jigs and transport devices used for real axle installation during manufacture would have required re-tooling to accommodate a new width rear axle so Ford just went with what was readily available.
This info comes from a Ford van assembly employee who was there during the early years of the '92 and later E-Series. I'll see if I can find the thread on FTE about this very topic.