I have an 1989 S10 blazer with a 4.3l V6. It has a G80 diff in the back and it's 2wd. Ever since spring started I started noticing the temperature on my truck starts rising when I'm at a traffic block or when I'm parked. It drops when I am on the move. In the last two years I've changed the AC clutch the heater core ,the water pump and the thermostat. I also checked the radiator hoses to see if there is any leak. There are none. It's maintaining the coolant level as well. I pressed the radiator hoses to see if they are pressurized after a drive and they are hard. I also changed the oil to synthetic around a month ago. I was wondering if someone could help me diagnose this issue.
Since you have a clutch fan up front and it spins freely, I'd suggest your clutch is fried ... which considering how old the vehicle is, doesn't surprise me. The way the clutch is designed is it has a viscous coupler inside which, when the fan becomes warm, will lock it in place and pull more air. Even when the engine is completely cool, the fan should still have some amount of resistance as you spin it. If it spins easily at any time, the clutch is done and needs replaced. You can either find a replacement clutch at a junk yard (ie: pick-a-part; breakers; etc) or you can usually purchase them new from the aftermarket for a reasonable price. Considering what you've stated, this seems very likely and replacing the clutch should solve your overheating issue.