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Here in Texas we have a problem with Rodents and squirrels, find a lot of wire damage from cars sitting outside in rural areas. Just a thought!
Turn your key 1 click, Before run position, do not turn ignition on. See if shifter will move. The brake switch has to work in order to release normally. If you can get it into neutral you can start the truck and tank it to someone to test it, unless you know how to test the brake light circuit.
p0203 indicates injector #3 control circuit, p0302 or p0303 is misfire codes for #2 .
Does your high mounted (Cyclops) brake light work, it's wired straight from the switch. The rear (tail)/brake lights go through the multifunction switch, (turn signal switch). Fuse #13 in the panel fuse box.
Who did the muffler work, ask them to look at it, if they did something to cause your problem they should help you with that.
Get your shifter linkage adjusted. It attaches to the neutral safety switch, not supposed to start unless it is registering park (or neutral) to close the switch so the starter can engage.
Pull it back down and check that your gasket is in place. TIP; I always use a sealer on any water seal, Just saying (no come backs).
It means you have a 5v reference circuit problem from the PCM. Ref cir #1 feeds the fuel tanK pressure sensor and the EGR. #2 feeds the MAP sensor and the a/c refrigerant pressure sensor. They send a 5v sig to each and the return voltage tells the computer how it is working or not working. See a tech to trace the circuit and repair it.
Have it pressure tested, and the cap tested.
It's easier with the right tools, which engine do you have and do you have any fuel line wrenches?