My pickup wasn't shifting into 4th except when it wanted to, and then finally wouldn't.  Took to transmission shop where they serviced it, checked all of the grounds, installed a new OD switch and a new computer programmed for it. 

Still wasn't shifting. After chewing on it awhile of a possible cause, he called Dodge to ask questions about this particular year and model of why it wouldn't shift. They told him that they had some in the past with those symptoms and ended up being the alternator needing replaced.

They unhooked the alternator, fully charged the battery and took it out for a test drive. 

The pickup shifted just fine like it was supposed to and stayed in 4th like nothing was ever the matter. 

They couldn't let me change the alternator on their lot, so I hauled it home and installed a new alternator from the original at 366,xxx miles - if it was the original- it sure looked like it. BTW, this transmission had new guts put in it 62,000 miles ago at the same shop and had no problems up until this started. This really had them scratching their heads. And they were very fair on their price for the work that they have done on it both times. I'm happy with them. 

With the new alternator on, it still doesn't shift into 4th, like it did before. It can shift up that "half" gear when I push the OD button, like before, and I can drive around town 45-50 mph without really overspeeding the engine and sucking gas. 

So, my question is this, what is the connection with the alternator having to do with the transmission not being able to shift into 4th when it's hooked up? Sounds like something electrical, but what???

Or, is there anything else that might have been overlooked or not thought about? 

Thanks for any input. Jim