When testing starter out of truck works fine, put starter in truck and under load starter stays engaged, rip

This may sound obvious but have you checked the battery? Batteries are generally considered old if they are over 5 years old. A battery can hold a full charge but struggle when put 'under load'. Starter motors use a lot of power to work. Get it "Load Tested" to see if it needs replacing.

you have a short in the system get a multi meter to test circuit

Do a load test on the battery - probably needs replacing.

Hi Mark
Sounds like you are doing all the right things.
We used to Load Test a Starter Motor by putting it in a Large Vice and connecting a known good battery with solid fat jumper leads.
When the solenoid throws the cog forward to engage we would use a 2 by 4 block of wood levered against the cog to try and stop the starter from spinning. The wood burns up under load but it quickly proves the Starter is functioning under load. Without using the engine.
Also you could try removing the spark plugs on the engine to allow the compression to escape. Make sure NO sparks around to ignite any fuel vapor.
The engine should spin over easily like this and you can hear & see if anything is binding up.
Hope this Helps!
Good luck!

At the starter relay, you have two red or purple wires, one goes to the starter and the other comes from a 30amp fuseable link. Is that fuseable link circuit hot ?
From the key switch, power goes thru the alarm relay, then the clutch safety switch or automatic selector switch, then to the starter relay. The fourth wire on the starter relay is a ground.
You should be able to put power to the starter relay on the white or yellow wire coming from the safety switch to make the relay engage.
My guess is something wrong with the alarm system.

Well since you replaced the starter, I think I would load test the battery cold and then again hot. It could be you have a dead cell in the battery and it is not producing enough voltage to work the starter solenoid.
Or it could be a bad connection between the battery and starter, or battery and ground.

Hello, There are small wires that attach to the Starter Relay. These are the wires Mechanics call "tickler" wires. There should not be power going through the tickler wire unless the Ignition switch is turned to the START position.

Disconnect the small wire off of the Starter Relay. Leave it off and put your battery cable on the Battery. The Starter should not crank. The Ignition should be OFF.

Now we need to find out where the tickler wire is getting power from. Put a Multimeter or test light on the Tickler wire for POS and ground the other wire on the tester to a NEG ground. I suspect your problem is in the Interlock switch. The Interlock switch would be on the Clutch pedal lever under the dash for a standard shift.

If you had an Automatic transmission, the Interlock would be in the shift linkage or Brake pedal lever. Without changing from an Auto to a Standard trans people have trouble hitting the under dash Interlock switch with their shoes getting in and out of their cars. Even while driving you can hit the under-dash switches with your shoes.

So check under the dash for a broken or dislocated switch on the Clutch or Brake pedal levers. Then move the switches until the Test equipment shows the power is not going to the Tickler
wire . Once you do this you have to make the START position work on the Ignition switch. Leave the tickler hooked up to the test equipment until you get the wiring right. You may have to adjust the Ignition shift-rod in the steering column so that START means the correct thing only in the start position.

I hope my solution is very helpful in giving you ideas about fixing your car.

No it doesn't mean that you have a problem.When you cross the lugs with a screw driver no matter whether you have the key on or not it will still engage the starter.The reason being that you have the hot wire from the battery going to one terminal and your ignition switch on one terminal.What your basically doing is bypassing the switch and directly energizing the starter.A good way to test a starter is to remove it from your truck hook up your battery cables to your battery.Be very careful when you do this.Hook the ground to the starter casing and just touch the hot cable to the battery lug on the starter.This should engage your starter.You might want to take a look at the fuel delivery system for proper operation.

Check the fuse box and make sure no fuses are blown. I had the same problem with my honda motorbike and thought the worst of it but then checked the fuses and found that one had blown. Once replaced the bike was fine!

Hope this helps you in someway! Good luck!

resistor.............tell the mehanic