Why will it not shift out of 1st ? on 2001 Dodge Ram 3500

It started shifting back and forth in between 3rd and 4th when around 50. Now it will not shift out of 1st. Tranny filter or Solenoid ????

Asked by for the 2001 Dodge Ram 3500
Soleniod would be recommened...... also i would tighten up the belts in the transmissionlook it up on youtube theres a guy that shows step by step
i would that and if that dont help then i would rebuild if i was in shoes