It's supposed to downshift out of O/D when load increases...... Is your TV cable adjusted properly?

While you are in the trans changing that spring, wouldn't hurt to change filter, adjust the bands, and have a look at governor pressure sensor/solenoid. (make sure they aren't sludged up.)

 

I just changed the fluid and filter a couple of weeks ago, hoping that would help. previous owner had it overfilled. This is my first Dodge truck and I am learning as I go.
I did not look at the gov sensor/solenoid, or adjust the bands.
I read about the throttle cable and tried to get it off to check it but couldn't see how to get it off. I didn't want to break it so I figured I'd just try to get the fluid level correct then figure out how to get the cable off & check adjustment. Haven't had time to get back out there with this crappy weather we are having.

 

And if you change the spring, upgrade. Don't just replace with OEM spring. Check sonax website.

 

I ordered one through Cascade Transmission. We'll see if it helps when it gets here.