It's likely that your line pressure isn't being reduced as it normally would be when coming out of lockup / 4th. Normally, pressure is lower when the trans is in lower gear, and rises as it progresses through the gears. But if it's prevented from dropping, and the line pressure remains too high, you get the harsh shifting in 1-2-3, as you have now.
The question will be finding out *why* the pressure is too high. There are a few causes, both inside / outside the transmission, so I won't detail all of them here, so that you don't go on a wild goose chase.
The good news is that if it's found / corrected, you probably won't need to fully rebuild / replace the transmission. If the issue is with the torque converter or the valve body, then you'll need to disconnect the trans from the engine, or at least drop the pan & remove the valve body.
As a temporary workaround, you can try the following...
After exiting the highway, find a place to pull over. Put the truck in N or P and shut it off, then restart it. You may need to experiment with how many seconds to wait (start with 10), but if you get it right, the pressure will return to normal when you restart the truck and engage the gears again, just as like when you normally get to your destination, shut the truck down, and restart it later. Cycle repeats when you get back up to lockup and stay there for a period of time at higher speed.
If you're just in full lockup for a few seconds, you probably won't experience the issue.
If that helps, you can continue to use that until you find a shop / mechanic, or you diagnose the cause, depending on your own proficiency as a mechanic.
You do have tow/haul mode in your transmission programming (button on the gear selector). That raises the line pressure by itself, so it won't solve your problem. But you could experiment with driving in tow/haul on the highway, and seeing if the line pressure drops afterward (or disengaging when you come to a stop, etc.) I will bet that it won't help. But it may be useful info for diagnosing the issue further (and / or relaying to your shop).