I think for starters I'd change the thermostat and back-flush the radiator, and maybe take a look at the water pump impeller to see if it may have spun.
I'm not sure the trailer had anything to do with the problem. Of course you could always hook up the trailer and tow it to see if it overheats.

 

 

If your truck has one. I have a92 f150 and it failed on me with a brand new thermostat. It should sit on your thermostat housing.

 

The 1984 - 1986 4.9L engines do have a coolant temperature sensor located in the thermostat housing, but it is for the computer.

 

That is right the gauge sender is on the right side rear of the block under the intake / exhaust manifolds.

Now if it was spitting into the over flow then it is not the sender it is HOT!
Get the motor up to temp. Shut it down and feel across the radiator face, edge to edge up & down side to side. You should feel the same temperature across it if a spot is cooler then you have no flow and a plugged radiator.

You can also look down inside thru the cap and if you see white crusty stuff around the tube that is what plugged it.
I pulled my radiator laid it flat and filled it with vinagur let sit 3 weeks drained and flushed with water and did it again with fresh vinagur.
It did not get rid of all the crust but did pretty good.
Dave - - - -

 

I forgot to say if the temp starts to go up turn on the heater full blast it will help pull the heat out.
You had the AC off when it over ed heated right?
Fan shroud on radiator?
Dave - - - -

 

First off, thank you all for the guidance on this. To answer a few questions for you, Dave... The truck came from the factory with no A/C so that isn't the issue. There is not a fan shroud so I will be ordering one of those regardless as I can only assume it will help or at least it can't hurt. I've been driving the truck almost every day and it has been fine. When I'm not towing, the temp stays well within the normal range. However, if I am sitting in traffic, especially in the Texas heat from hell, the temperature will rise to the "L" in "Normal" and will begin to vapor lock once I get moving again. Once I am moving again, it corrects itself almost immediately and stops the vapor lock and cools back down. I haven't had a spare minute due to work being so hectic but I am going to try and start getting to the bottom of this sometime next week. I'll be sure and update everyone with my findings. Thanks again!

Brad

 

The shroud will most likely fix the getting warm when idling part, and as you say, cant hurt the rest of the time. Getting hot pulling such a light load is odd, does the trailer have a lot of wind resistance or something? Something else to consider, the viscous fan clutches do wear out, and as they go they engage later and later till they dont at all. With the vehicle cold try to spin the fan by hand, it should turn, but very stiff. When you first start it, rev it up a bit, you should hear the fan working, then i should quiet around 10-30 seconds.