Who knows Dodge Ram auto.Transmissions?

This may sound silly but you may be low on fluid or have to clean your filer out again and make sure to jack the back tire's off the ground and let it free spin though the gears.
If it was dirty your clutches may be gone, or you just need to adjust the bands if there has been worn.

But!!!! The valve body may be sticking ! did you clean it out real good.
there will be some good tec's jump in here soon.
I had a 727 rebuilt and drove it home and the next day it did the same thing
and I called the rebuilder and he said to drop the pan and bring him the valve body, He found a stuck spring and cleaned and gave me fluid and gasket and filter an 50 bucks and it never gave me another problem.