I have an electric, cordless, and high quality pneumatic 1/2" impacts. I would rate them int hat order from weakest to strongest. Two weeks ago I had a flat on the front of my backhoe and the pneumatic impact could only get two of the bolts (they are bolts not nuts) free. The rest took a 5' section of pipe on a breaker bar to free up. But the bigger question is are your tires filled with ballast? If so even if most leaked out that tire is still going to be very heavy and hard to deal with. Then once it's repaired you're going to have to have the tire guy come back out and fill it. If it's not filled it'll still be heavy but with help you can handle it. Getting it into the back of a pickup could be hard.
I think the first thing you need to do is determine if it's a tube as others have said. If it is then you'll need to get the tube out to repair or replace it. You'll also want to get any rust or dirt out of the tire before putting the tube back in. If it doesn't have a tube then you might be able to get away with something like tire slime (if it doesn't have ballast in it). If it does then you'll need to remove the ballast and use a bead sealing product or tube. A rear tire is not something I would suggest doing unless you have plenty of experience with tires. Even though the pressures are not high there is a lot of air in the tire and it can cause lots of bodily harm if something goes wrong. There's a reason why tire shops have a cage to put large tires in when they inflate them. Better safe than sorry, a trip the the emergency room would more than offset the cost of paying someone skilled to do the job.
