My Dakota won't move in reverse

I recently started having an intermittent problem with my 04 Dakota 4x4 when I put it in reverse. It goes into reverse and you can feel the truck wanting to go backwards but it will not move. It sounds and feels like something is binding up somewhere. I'm not sure if it is the trans. or the rear end but it feels like the rear end. It has been doing this once in a while lately but usually I can get it to correct itself by putting it back in drive and going forward a bit and then backing up. I usually back into my driveway so I don't have a problem being I am moving forward when pulling out. Today however I pulled in forward and now I can't back out as I can't move it forward any further to try and free up whatever is happening. The truck has only 35K on it and I don't drive it very often (maybe once a week). Has anyone ever experienced this or have an idea what could be causing it? Could the fact that the truck is sitting most of the time without being driven have something to do with it?

Will it roll backwards in N?If it does that should elliminate the rear.Ive had a few Mopars that lost R.I changed the fluid and filter and added a bottle of TransFusion.Out of 5,4 worked just fine.Rocky

What engine/transmission?

I haven't tried rolling it backward in neutral. I will try that tomorrow. The engine is a 4.7 and it is an automatic 4x4.

Where can one find the "transfusion"?

I tried putting it in neutral and rolling it backward and it won't move. Then I jacked up the rearend and put it in reverse gear. The wheels move just a bit and then lock up. They spin freely when in drive.

Bill I got it at Auto Zone.Its the one additive that actually works.PepBoys did carry it also.I believe Solder seal makes it.Ive used it since the 70s.Rocky

It sounds like maybe the pinion nut came loose on the rear end.
If the pinion has too much play it will slide into the ring gear in reverse and lock things up, but it should have been noisy or growling going forwards too.