Transmissions wont go into overdrive

I got the truck cheap cause they said the tranny
was shot. Bit it looks brand new. So we flushed it
and put New fluids and filters in it. Still wouldnt
shift on its own. Have to start in second then to
drive when u get to speed. But will not go into
overdrive. So my RPMS are extremely high. I put
some sea foam in it and its shifting almost perfect.
But still will not go i to overdrive. Sometimes when
i start the truck its in over drive and wont shift out
of it for nothing. I have to pull over and shut it off
and turn it over again. I think it may be the tow/haul
button on the shifter. Just guessing though. Any
suggestions would be great