1997 Caravan transmission sticks in neutral

My 1997 Dodge Grand Caravan 3.8L began to have the following symptoms:

1. After the van sits over night, I can back out of the driveway in reverse with no issue.
2. When I then shift into drive. It appears to instead go into neutral and then will not shift to any gear (Reverse, Drive, Low, or 3)
3. Usually after 5 – 60 seconds it does engage into drive and go to work with no further issues.

During this issue I have tried:
Shutting the vehicle off/on. No effect.
Revving the engine: Possibly helped, but seems more likely to be coincidence, as it often also engages without revving.

More info:

– No trouble codes displayed when using the “Torque” app for android.
– No “check engine” light.
– This Van was parked, unused for about 9 years with new fluids. When I put the van back into service I changed oil, but not transmission fluid.
– The transmission fluid looks normal (clear and red). I’m planning to change the fluid today.
– Before the van was parked I replaced the CAM shaft, so I had the entire engine out. The Van has been fine for the first 400 miles since putting it back into service.