It's possible that you have a vacuum modulator problem. Shift pressures are modified by modulator pressure. Modulator pressures are created internally in response to a valve moving under the influence of vacuum on the diaphram of the modulator.
The modulator is a plastic device (either red or green probably) on the left side (driver's side) of the tranny. It has a vacuum line that runs to the engine intake manifold lower section rear. Verify that the line is hooked up at both ends. If it is, a gauge should be teed into the line at the moduator and vacuum verified.
If you have vacuum the next step would be to test modulator pressure under different vacuum settings. The modulator pressure should be somewhere around 55psi with no vacuum at high idle. It should drop to probably around 15psi at idle vacuum (plugging and unplugging the line to the modulator will give high and low vacuum for these readings).
For this to be your problem the modulator pressure must stay relatively fixed during these tests.
Bosch Master, ASE Master, L1
33 years MB technician