Well then I think we are looking at one of two things that might be causeing this...
Shift Solenoids, located on the solenoid block on the valve body
OR
MLPS, Manual Lever Position Sensor... AKA Range Sensor.
You can safely change the range sensor yourself and it's not at all overly involved. On the V6, you will have to remove a few things out of the way to get to it, but other then that it's no problem. You can get the sensor from Bill Jenkins for about $21, plus shipping. You can find it
here.
As far as replacing the sensor, you would place the car in drive. Off of course! You will see on the sensor that there's two marks that line up on top of the sensor. You then unbolt the sensor and install the new one. Make sure that you line the marks, located on top of the sensor, and bolt it down. Once everything is secure, you move your shifter to the park position and back to drive. Recheck the sensor to make sure the marks are still lineing up. If so, you are good to go. This is sort of failsafe, so don't worry.
As for the shift solenoids... This assembly will run you about $160. It also requires that you remove the valve body cover. Which on a V6 means that you will have to remove such things as the battery, battery tray, things like that. This one is a little more involve, since you will be digging around on valve body. so if that make you uncomfortable, then I suggest you take it to a shop.
I will say this as well... There could also be a problem with the pistons that shift solenoid control. They could be stuck or leaking inside the valve body. If this turns out to the be the case, you will have to replace the valve body. We have had a lot of luck from a company, that I cannot think of right now, that sells remain'd valve bodys for a fair price. Plus, they are all rebuilt with Sonnax Master Rebuild kit, which addresses a ton of known weakpoints in the CD4E. I will try to hunt down that information and post it.