Pulling the DTCs will provide you the fastest answer.
Has the check engine light illuminated?
Limp-home mode shifts the modulation pressure to maximum, which is why you are experiencing the abrupt gear engagement.
Several things can cause the transmission to enter limp-home ...
- the wiring harness plug adapter could be leaking... an upgraded adapter with improved seals was introduced with MY '98 production. Leaking adapters tend to throw two specific codes (I believe one is 113?) which, when seen together, can indicate a faulty adapter. The part is cheap ($10) and is not difficult to install. Visual inspection of the existing adapter can be performed if you don't have ready access to a scanner. The transmission's wiring harness enters the transmission at the right front... simply rotate the locking collar, remove the harness and inspect the interior of the plug adapter. If there is ATF present, the adapter should be renewed.
- the speed sensors integral with the electronics plate attached to the valve body may be defective or coated with so much metallic debris that a signal can no longer be detected. A new, improved plate is available. Ensure that a fluid pan magnet and new style plug adapter are installed at the time of the plate's renewal. Again, a fault code check will be necessary to pinpoint a defective plate - codes will indicate implausible or occasionally implausible speed sensor readings; ME fault codes may also be present, referring one to check ETC.6 fault codes. Removal of the valve body is required to renew the electronics plate.... not a technically difficult job but cleanliness is imperative.