I have a 1993 Aerostar.  Although I have never towed very much with it,
with all the kids and their stuff, and using it for vacations, it
usually carried a good load.  I always kept it well maintained, changing
the fluid at regular intervals.  It started slipping at about 115 K
miles, which according to what I have heard from many different sources,
including some friends that work for Ford is that this is about the
expected lifetime for this trans.  I went with a rebuilt from Ford for
about 2 grand and now have over 180 K on the van (installed it myself).

The problem with a used trans for this is finding a unit with low enough
miles to make it worthwile.  If you really like the van and intend to
hang on to it for a while, a rebuilt may be worth the admittedly extra
dollars.

Bob