Starter wont turn over

Underhood in engine compartment. You mean dash lights go out or dim?

You have conflicting symptoms. If it won't even crank, how can it sound like it's out of gas? You should hear the fuel pump run for one second when you turn on the ignition switch.

Get the battery load tested and make sure its clean and properly tightened-changing plugs and wires won't do that unless you touch something you shouldn't have-backtrack your work

Do you have a security system if so have that checked too.

It may be wired the same but where it connects to the piece going in the truck may be different. But going from a 4 to a 6 you neeed to also change the computer as there may be a big differnece there, especially to make it run.

Pick your symptoms below check it and get back with some testing results

**Cranks but no start condition:

Get a helper disconnect a sparkplug wire or 2 and ground it to the engine at least 3/16 away from ground -have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel related problem, Do you hear the fuel pump come On when you turn key on? If not check fuel pump fuse and fuel pump relay if so, check the fuel pressure to rule out the fuel filter/fuel pump/pressure regulator and listen to the injector/s are they pulsing or hook up a noid light. No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs, coil's resistances, distributor pick-up coil, ignition control module, cam and crank sensors and computer Note: If it doesn't apply disregard it.

**No crank at all/nothing/nada when key is engage to starting position

Could be a blown fusible link, the starter, starter relay, clutch switch/park and neutral switch/transmission position switch and ignition switch assuming the battery condition and connections are good. Note:If it doesn't apply disregard.

Are the battery cables in good shape? Have you made sure the terminals are clean and tight and the ground at the block is good?

I'd have to be sure the battery has a full charge and all connections that lead to and from the battery are ok. Next, I'd check the circuits at the starter relay. Terminal 30 has voltage/hot, all the time. Terminal 86 goes hot with the key in the start position. Terminal 85 is ground for the coil side of relay, grounds through the neutral switch. Without voltage and ground the relay won't energize.

Check ASD relay in the relay box (mark it and switch it with a similar one )

check ignition coil very common too

Does the starter make a loud clunk? Do you hear the starter relay click?

That implies the neutral safety switch and ignition switch are working. If there's no loud clunk from the starter motor, the logical suspect would be the relay but they aren't a real high-failure item. Switch it with a different one like it in the under-hood fuse box.

Put your finger on that relay while a helper turns the ignition switch to be sure it's really clicking. There are other relays that will also be clicking so sounds can be misleading.

You can reach over the left front tire to get to the two wires on the starter. If you carefully short them both together with a screwdriver, the starter should crank the engine. If it does, that will verify the battery and cable connections are okay. If that works, remove the relay and jumper terminals 30 and 87 with a piece of wire or stretched out paper clip. That should also make it crank the engine.