I've heard that changing the trans fluid in a car that
hasn't had it done in a while may lead to this. The
old fluid keeps the sludge (for lack of a better term)
in places like clutch packs which may have developed
a small leak. The new fluid flushes out the old and
the result of wear becomes more obvious. This is
just one possibility, especially on a high-mileage trans.
The easiest fix to try first, besides the correct level
of fluid, is a simple adjustment. On the driver's side
of the trans is a round piece protruding about 3/4"
with a vacuum line going into it. In the center of the
(~3" dia.) modulator, as it is called, is a rubber cap.
Pry it off, and use a small pliers or similar to carefully
pull out a little tee handle. Visualize its current position
before turning it, and just pull it out enough to be
able to turn it. By turning it clockwise, the pressure
is increased and the shifts become firmer. You may
give up other shift qualities but this is one way
that might take care of the 1-2 indecision. Try turning
it one full turn cw and see how the trans behaves.
You can turn it more if one turn doesn't quite do
it. There are many threads about getting the correct
vacuum signal, which would be the next step if
this doesn't work.
Adjusting the trans due to wear does not necessarily
mean it is worn out. If you do decide it is worn out
(hopefully not too soon) then you can use Lucas Trans
Treatment/fluid. It is like 90 weight trans fluid and the
extra viscosity slows the internal leaks enough to
make it shift close to normal. Good luck
Mark T 82' 300SD 280k