You try to pry the hinge off of the tailgate sideplate and the hinge bends, but doesn't come off. @#$%$%^^&&!!
I googled around and found that many others have had the same problem. The most common solution seems to be to grind the old hinge away, then install the new hinge. The problem with this "Grinding" solution is that there isn't a lot room where you need to grind and consequently, you run the danger of the grinding wheel cutting into the tailgate in bad places.
Some people say that that single bolt will work just fine to hold the hinge in place when you install it, but others say that it HAS to attached in two places.
A less frequent approach is to flame cut the weld out. Not good if you want to save the paint.
Drill out the plug weld.
This approach doesn't use any heat, so paint isn't destroyed.
No grinding is involved.
Drilling out plug weld gives you a hole for a second bolt when you attach the new hinge.
Look at the hinge from the "outside". Note the "cup". Yours is probably rusted pretty badly or you wouldn't be reading this.
Remove the single bolt by whatever method works for you. I choose to spray the bolt with PB Blaster and wait a day or two before attempting to remove it. My bolt (which is metric) came out okay.
Now drive a chisel in behind the bracket to bend it away from the side panel about a half inch. This won't take the hinge off.
Now look at the back side of the side panel and examine the plug weld.
See that it is a "hump" of metal, thickest at its center. Noting the location of highest part of the hump, locate that same point on the inside of cup of the hinge. Center punch that point.
Drill all the way through the cup and through the plug weld with about a 3/16" drill bit. In my case, there was no problem with excess hardness in the weld.
Now enlarge the hole to 5/16" diameter. At this point, you should have drilled out most, but not all of the weld material. Take a hammer and tap on the part of the bracket that you bent outward. If things work you as they did for me, the bracket will twist, showing that the weld in almost completely gone. After that, all I did was to rotate the bracket back and forth a couple of times until it fell off.
If you can't get the bracket to rotate at all with the hammer, enlarge the hole slightly more until it does. I'd go in 1/32" increments.
Intall the new hinge and tighten the single bolt. ***At this point, I put some never sieze on the bolt so it doesn't get stuck now that I am so close to success***. Note that the hole in the hinge is elongated. This is so you can align the tailgate so it closes properly. Gently put the tailgate back on and adjust the hinge location as needed.
Remove the tailgate carefully, so you don't disturb the hinge.
Now, with a pencil,from the back side of the side panel, mark the location of hole you just drilled onto the back side of the new hinge.
Remove the hinge and drill that hole out to be about 1/32" bigger than the hole you drilled through the tailgate side panel.
In my case, I had used a 5/16" drill to get the plug weld to let loose, so I put a 5/16" X 1" long hex head bolt through the hole in the hinge and fastened it with a helical lock washer and nut on the back side.