727 experts? Need help

You are going to need to pull the pan and find out why.
It's pretty easy to diagnose once you are seeing it working, and we/I can help with pictures and descriptions.

Like I said earlier, everything for the park function is in the tailhousing except for the rod.
When the VB goes in the rod has to get pushed through the park assembly.
Then when the linkage is put in park it pulls the rod forward until the knob on the end forces the pawl into the gear and locks the output shaft resulting in park lock.
Also when the pan is off you want to make sure the linkage arm is seated in the manual valve (common for new people to miss) and it won't change gears but WOULD still go into park because the rod is not part of selecting gears.

Check these pics out.