Frequently, the problem is the torque converter clutch is failing to lockup. Which is why it's kicking out of OD. My Mustang has the 5R55 trans, and it was doing that to me a lot last year. Replaced the fluid and filter and all the solenoids. It was fine for awhile. Then a few Saturdays ago, drove it to work. Came out to go home, started it, but nothing happened when I put it in gear. It's in a friend's transmission shop now, and he's scratching his head over it. Says the 5R55 is the worst transmission Ford ever made.
A transmission shop will have a code reader that can read transmission codes. Most store bought one's can't, unless you buy a real expensive one. The code I was having was P0741, "
P0741 OBD-II Trouble Code: Torque Converter Clutch Circuit Performance or Stuck Off
"
If your truck has a tach, you can see if it's locking up or not. Over 40 mph, the tach needle should be very inactive in OD. If you lightly put your foot on the gas and the needle moves, it isn't locked. Same for backing off the gas, it should not move. Should stay pointing at a speed and barely move when you move your foot. Overactive tach in OD over 40 mph, the clutch isn't locking. It doesn't really cause any problems, but will make the trans run hotter than normal.