My 99 Trooper all of a sudden started having shifting problems and acted like some of the transmission positions were not available.
It turned out to be the electronic shift module the connects the transmission to the floor shifter.
This module can be rotated for "fine" positioning of the shift locations.
Purchased this GM/Isuzu module and installed it myself, cost over $125 for the part.
It took a few times to get this right, this happened kust after it hit the 100K mark and a long trip through the mountain in old mexico when quick down shifts were required for steep down hill grades.