I would take the governor out and clean it up. Do you have reverse? If yes then it is just dirt someplace. I have talked to transmission builders in the past and they say to drive about 40 MPH and try to put the trans in PARK!!!?!? You are not to let it go into park just push it up and then back to drive fast. They say it will rattle the daylights out of the transmission and knock loose whatever is causing the no up shift. Now I have never had enough of what it takes to do this, like money, but if it works great. For me I would just spray the governor and move the weights and spray again and then reinstall. You don't need to drive it around just raise the wheels off the ground and put it in gear. Give it some gas and watch the speedometer. When the truck shifts the needle will jump. Also see if you can make it shift manually. Put the selector in "L" and then push if up to "2" and then when you think it should shift into high push the selector into "D".
The last thing I would do is put some type of transmission cleaner into the oil and drive it around and see if it will free things up. If all else fail to cure the no 3rd then and only then would I pull the trany for cleaning.