If you have a lift and stands to hold and lower the gas tank then going at it from under the vehicle is the preferred method.
If you don't then raising or sliding the bed back is the preferred method.
The bed has 6 bolts that hold it on, you also need to remove filler hose(on either method), and then unplug tail light harness.
You need two people to lift and move bed backwards enough to access fuel pump ring.
Some just lift the bed in the front and prop it up to gain access.
Good look here, with pictures: How To: Replace Fuel Pump
To lower the tank you need to remove skid plate, if so equipped, and then the filler hose and tank straps, then lower the tank enough to access the electrical and hose connections on the top, remove those and then lower tank down and out so you can lift out old fuel pump assembly, the assembly is the same height as the fuel tank, so you need at least that much room above the tank to get it out.
With this method you can also clean out the tank, flush it out, flip it over and let it dry, if you feel that is needed.
You can also test if it is the Check valve that is leaking pressure by clamping the fuel line near the engine, you need to be careful about the force you use, lol, don't want to crush the hose.
Get clamp ready, vise-grips work, but again not too tight.
Hook up pressure gauge
Turn key on and off a few times to build up pressure
If pressure seems to hold, release clamp, sudden drop means check valve is leaking