Here is a little added background of what I went though with my cooling system. I had the same overheating only on the freeway issue for almost a year.
Upon purchase of the Jeep I upgraded to the high flow water pump only because I though the original was toast as I had constant squeeling. No cooling issues. I realized later it was just a cheap belt.
About a year later I added the triple fan kit and some Napier hood vents. Still no overheating issues.
About a year after that I started having overheating issues on freeway. I began continuely trying to burp system. Temp would quickly drop and hold at 210 at idle, but quickly climb at 55+ mph for more than 5 minutes.
Short tips to work or around town, no problems. Any freeway driving or hills and temp would quickly climb. I fought with this for over 6 months. Thowing new hoses, t-stat, burping techniques, filling system though upper hose, etc. Nothing worked.
I finally replaced the radiator, only thing I had not replaced. That weekend I wheeled at Tierra Del Sol, Southern California desert with temps in the 90's. Jeep never broke over 210 all weekend.
About two months later, the over heating began again. Only at freeway speeds again. Now I was stumped as I had replaced everything. After playing the "burping" game for a few more weeks, I figured it must be the cylinder head. I had the head rebuilt and replaced the head gasket. Everything back together and first freeway drive, it overheats again!
This is when I started back at the basics and did the Super Flush. It took several flushes to clean it out, but after each flush, the temp would climb a little slower on the freeway. Once the system started flushing out clear, temp never climbs past 210.
Moral of the story, start with the simple stuff before throwing a ton of money and time into swapping parts.
Edit: Oh yeah, I also completely rinsed out the overflow bottle which was caked with crud on the bottom. Probably the most crucial part of the Super Flush.