Shifting in and out of overdrive
 

My 2001 Ram 2500 4x4 diesel shifts fine on normal take-offs from a stop, but when I have to ride steady at 45-50 mph it will shift back and forth between o.d. and 4th gear, especially going from no-load easing along to slight incline load. If I have the space to go ahead and accelerate, it speeds on up without this toggling between 4th and o.d. Not knowing much about automatic transmissions, I'm wondering if it could be some kind of vacuum problem. Any idea what can be causing this?

Welcome to DF
My guess is you may have a bad TPS or APP sensor or the brake switch is kicking you out of lockup. But i would go with the TPS first, which is a common problem. I will move your post to the Diesel section. You'll get more response there.

Thanks for the reply. I'll check those first.Also thanks for moving the post where I can find it easier. I'm new to the forum,and having trouble returning where I left off.Searching username doesn't always get me there.

I think my brain locked up :) I know what the TPS is, but how about the APP sensor? and aslo, where are they located on the Cummins 5.9 ?

Check for bad grounds, these things tend to freak out if there is any kind of electrical anomaly.

My 3/4 ton gas motor is doing the same thing. Also it idles up and down when ever it wants to so I was wondering if there was any resolution with the tps.

APPS = Accelerator Pedal Position Sensor

Had the same problem an my 07 2500 5.9 Quad Cab (Now 200.000 miles). It happens every 50.000 Miles aproximately. My truck is usad mainly on highways or plain offroad (Constuction sites), but normally towing or hauling big loads. It is due to misadjusted belts in the tranny. Now I send it to a service (oil+filter+belts adjustment) every 50.000 miles and the problem has always been solved. Hope this helps.