When starting, the truck starts but immediately turns off, the only major difference is te security light doesn't go off, it then starts to blink. I have an aftermarket remote start.

The red light flashing is telling you that the security circuit has not recognised your key chip so it will not start. Either there is a fault on the reader or you have dropped your key and damaged/lost te chip.
Ring an auto locksmith. They will quickly cut you a set of keys and code them to your car.
Always keep a spare key handy.
Roger

How about the ignition swich? They have been recalled on this model I think, call the dealer with your vehicles VIN and find out.

try a different key, the light will come on and stay on if the tumbler is not reading the resistor in the ignition key.
if that doesn't work try to start car, then don't turn key off leave it in on position for about 10 until the security light goes out, turn key off then try to start again.

Security is set. Lock the car up. open it with the key and turn it on. this usually resets the security. If it doesn't use the key fob while its still on and unlock it.

Hello: Will try to give you instructions: Attempt to start car, leave key on (security light flashing) let set for 10 minutes, After 10 minutes security light out. Turn key off for ten seconds, turn key on, let set for 10 minutes, turn key off for 10 seconds. turn key back on let set 10 minutes, turn key off for ten seconds, the engine should now start and stay running.

1. Attempt to start the car, let it stall, but DO NOT turn the key off! Leave it in RUN.
2. Wait 10 minutes or until the Theft System light glows steady instead of blinking.
3. Do not turn the key off, but start it immediately.
4. This should reset your Passlock system.

The other issue is that the BCM harness is known to corrode and will cause the passlock to fail and will lead to a no start. Check the BCM under the glove box, the bcm had somehow get's wet and the terminals corrode. Sand down all the corrosion that you could and put the bcm back into place and the car should started up fine and worked normally if done right. ONLY DO THIS IF YOUR SECURITY LIGHT STAYS SOLID AND DOES NOT RESET FROM THE ABOVE INSTRUCTIONS.

Thank you for using fixya and keep us posted.

You need to reset the security system. Try to start it. The light will blink. Leave the key in the run position until the light stops blinking. It will take about 11 minutes. Shut the key off. On the next key cycle the system will reset and the car should start and run.

try a different key. if not probably a ignition lock cylinder. his is a pass lock security system. these have alot of problems with the two little wires that read the resistance from the pallet on the key code. Take off the steering wheel and visually check for damage two those wires they are white wires with orange coating.

try puting te ignition key in the drivers door lock an unloack it 3 or 4 times then try agin if it dosnt work, key needs reprormming

when u see the security light blinking and it wont start u simply turn the switch on and leave it on till the security light stops blinking then turn the switch off and start the car up