Need Help!

I have a 1989 Toyota Pickup 3VZE. The problem I'm having is the transmission is not shifting into 4th gear or OD until driving for a while and then it'll shift back into 3rd while driving flat. I'm aware that the truck needs to be up to operating temp for the TC to lock which makes me think I have a warming up problem. I drive the truck an hour to work every day and the temp gauge never shows more than 3/8 and it doesn't get that far until a good 45 mins of driving. It's the same way in the morning or afternoon, around 75-95 temperatures right now. I bought the truck not running so I've replaced the radiator and t-stat maybe 2-3 months ago. I don't hear any air in the system and I've elevated the front, took radiator cap off and left truck running with heat blaring twice just to make sure there was no air. Any inputs will be appreciated...this has got me stumped.