Not sure what is causing your issues. I know on the Stratus, it was the shift solenoid causing the 4~>3 bump shift.
As far as your Durango. What...
I haven't had it go out in our D yet, but I did have to replace the solenoid on the Status. Wasn't that bad.
Rockauto.com was where I got the pack from. It was even a Mopar product and cheaper than getting it from Dodge and the other auto parts stores.
I'm not 100% sure where it is on the D, but now you have me interested looking for it, "just in case"
2002 Dodge Stratus Sedan 2.4 DOHC 159k miles died 2/12/2013 with 178k miles.
Wife's DD: 2005 Dodge Durango 4.7/3.92/AWD 66k
My Toy and current DD: 2001 Camaro SS LS1/6speed