Hello and welcome to JustAnswer!
The P2146 is for bank 1 injectors being shorted. The injectors are broken up into two banks electrically, bank 1 is the front 3. This code will set from an internal injector problem, the valve cover gasket/injector harness, a wiring problem between the valve cover and ecm, or the ecm itself.
Start by removing the valve cover, unplug the injector harness for the front bank where it plugs in at the valve cover gasket/injector harness connector and remove the harness wires from the front three injectors. Using an ohm meter check across the terminals of each injector one at a time. Each injector should have a resistance of 0-1 ohm. If the resistance is higher than one ohm it will set this code, and that injector needs to be replaced. An injector is by far the most likely cause, followed by the valve cover gasket/injector harness that you've already replaced. I've never seen an external wiring problem or the ecm cause this, but it's possible.
Remember that this fuel system is fully self bleeding, opening a fuel line while the engine is running or cranking isn't necessary, and is very dangerous. The fuel is under enough pressure to penetrate your skin.
There are special tools but you will probably be ok without them. You'll have to pull the fuel line from the rail to the head. Once that's off you will remove the 15/16" nut at the head. This holds the connector tube in that goes between the fuel line and the head. There is a tool that threads onto the end of the connector tube to pull it out (it snaps in), but you can probably pull it out by hand, or gently grab it with needlenose pliers and pull it out without damaging the threads.
Then you'll pull the exhaust rocker arm out of the way and remove the two 8mm bolts that hold the injector down. There is a special tool to pull the injectors. There are clamshell pieces that go around it, a bolt to act as a pivot, and a handle that attaches to the clamshell and pivots off the bolt for leverage. I've normally had decent luck with grabbing the metal body of the injector with adjustable pliers and pivoting off of a rocker arm to pop the injector up and out. It's not the best way to do it, but it will get the job done.
When you get all done remember that it's fully self bleeding, all you have to do is crank the engine and it will start, don't open any lines to bleed them.
Thanks for the help!
You're welcome, and good luck with your truck!