when driving the truck acts like it stays in first on 2001 Dodge Ram 2500

but when i let off the gas pedal it shifts what is it

Asked by for the 2001 Dodge Ram 2500
could be a couple things from shift solenoid to bad fluid and filter. Service it first with new fluid and filter i also reccomend having it flushed to get all the trash out of it. your local transmission shops will usualy do both services for around $100 . and dont be afraid to ask them if there was trash in it or how the filter looked this way you know where you stand
we all ready had it flushed and new fluid addded but it still does it where is the solenoid located