1995 Ford Aerostar Van 2WD 3.0L 4 speed AT 188K mi. Fluid full and no
color/odors evident.

Late shift from 1st to 2nd gear, engine revs too high before shifting.
No problem from 2-3 or 3-4. Question: what is the best way to
troubleshoot the problem and identify/fix any non-internal
transmission problems (vacuum leak? solenoids? etc) before heading to
the local trans shop for the predictable advice "you needs you a new
trans-mission and its gonna cost you big" - Thanks

That sounds like another problem my family had with the 1991 Aerostar.
 It was high revving and slow going from 2-3.  The transmission folks
said that 3rd gear was gone, so it wouldn't kick in until 4th gear. 
It wasn't a huge problem since it was a higher gear and it was just a
work car, but it may be a bigger issue if it is a lower gear.  After
reading this post, I remembered that it was a vaccuum leak that was
part of the problem, too.