Have you tried flushing the radiator? I know on my 86 with stock radiator you do see coolant moving. I would guess plugged up or the water pump is getting tired.
If I remember correctly, when I ran a hose through the radiator when I had it out a while back. There was flow but I dont remember how much. I have seen some suggest to take it out and fill it up with water with the bottom outlet plugged. Then release the bottom plug and water should shoot out around 9 inches or so. I didnt know about this at the time but may end up doing that if the sending unit is good.
Ok, just got back from picking up the voltmeter. I will be checking the temp sending unit next but will have to save that for tomorrow.
Earlier, I was able to crank and immediately drive the CJ around the block (almost 1 mile) and by the time I got back the temp gauge was almost pegged on hot. Shut it off and checked the radiator cap a couple minutes later. No pressure noise, ans coolant inside was luke warm. But the top and bottom hoses and heater hoses felt hot.