new to the forum.
I have the same problem,not shifting back into park with my 2004 WG grand cherokee.
I have found the problem,under the centre console to the side of the shift lever there is a electrically suplied air (or motor) driven servo,not much bigger than a cotton reel,this is to stop you going into park while in motion.It is speed sensed,anything over about 4 mph and it locks you out of park.
the problem with mine is that it activates when you pull away but does not de-activate when I stop,hence I can't get back into park.
So on mine the centre rod of the servo comes out to 'lock off' park,but does not retract again.I am currently trying to track down exactly what it is called so I can get a new one.
I have removed it from the car so I can still use park,but just have to be carefull not to throw it in park while moving!
all the best