The JEEP GRAND CHEROKEE (1993-1998) had a similar problem which was then recalled.
The vehicle could shift suddenly from park into reverse, even when the engine was not running.
A hidden internal shaft lever within the transmission can actually land on a flat space between reverse and park, leaving the car in hydraulic neutral. From this spot, referred to as "false park mode," slight vibrations or movements can allegedly cause the Jeep's lever to slip into reverse gear and the Jeep to move.
DEALERS WILL INSTALL A SECONDARY DETENT SYSTEM TO THE FLOOR SHIFTER ASSEMBLY.