So this new to us van has a receipt for autotrans rebuild about 25k ago.

We picked it up and drove it the ~200 miles home..  Acted a tad odd that
didn't want to kick from 3rd to 2nd on a big hill.. But chalked that up
to driver interference and only happened once.  Consequent down shifts
were on demand and predictable.

Drove it a few hundred miles around town,  lend it to my father to get a
safety inspection and on the highway drops outta 3rd into 2nd..  @
~60mph...  Revved like hell and slowed down, took 2ndary roads rest of
way home.. 

So it'll shift into 3rd..  But only with no   that's ZERO throttle
action..  Touch the throttle and kicks into 2nd..

I've the Bently  and reading thru the adjustment procedures, but this
doesn't appear to be for a won't stay in 3rd scenario, but for a kick
down setting.. I've hypothesized that it's related, but the linkages all
appear to be in order.. 

Here's the PO story, he only had it for short time.. Wasn't into camping
as much as he'd thought.. 
He'd replaced the Throttle cable, and corrected an adjustment in the
linkages for 3rd to kick down better.. 
He'd ordered a screen & gasket kit.. (I have it now) just hadn't gotten
around to servicing the trans fluid.. 
There's A LOT of talk on Thesamba about type'F' as a
corrective/aggressive fluid change..  
It DOES NOT SLIP in gears, and only on cold occasion takes a moment to
build up and lock in the Torque converter on a 1st morning go.

So in hind sight the Pos story makes it appear he had ideas the trans
was flakey.. And I do have a spare Autotrans here in the garage I can
swap in a day or two..  But I prefer not to take knee jerk reactions.. 

I plan on dropping the AT pan, inspecting the insides, changing the
screen and adding new ~75K valvoline ATF dexron fluid.. 
I suspect I may find bits of RTV from a pan gasket that would really gum
up the werks.

Dan in NY