I have a 97' 3.9 magnum with a few billion miles on it. Shortly after I got it the tranny would "stick" in various gears but with a little kick on the gas it would smarten up and behave. This problem has gotten much worse (3 years later) to the point where it shifts into high gear when you put it in drive while still stopped in the driveway. It goes into high and stays there all the time...But here's the twist, if you manually shift it into 1st it will go into first and then you can manually shift to 2nd and then to drive and it feels good. So i know the tranny actually works!

I suspect electrical, it "feels" like the tranny thinks the truck is at speed and doesn't know that it is not moving. I have found that I need to switch off the overdrive and manually shift it into 1st when I am slowing down at a red light, if i don't do it at about 3 miles an hour it will not shift into 1st it will only go into 2nd and only when the tranny is cool. After it's hot there is no more 1st, only 2nd

Anyone seen this before? vehical speed sensor...?

 

If your 3.9 is like my 5.9 (I think their close) then there's a good chance you have a cable that leads to the TB that needs to be loosened/tightened up a little bit. I can't remember the name of the particular cable but it is right next to the throttle cable. Mine was too tight (I believe? I let my mechanic work it out since I have a warranty on all tranny issues since they rebuilt it) and would downshift too far if I got on it hard after cruising. I imagine your cable may just be too loose...