ECM fuse keeps blowing

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If fuse 4 is the one blowing it isn't the ECM fuse. The ECM is fuses 56, 12 and 5. The new issue of not running without you stepping on the gas was caused when you removed fuse 56. That reset the ECM learned behaviors so now it has to be driven a while to relearn the idle and shifting. However the original issue won't allow that as you cannot drive the truck like normal.
Fuse 4 powers a few items, the good thing is you should be able to discover if it is one of them that has failed by simply unplugging them one at a time. To save on blowing fuses you can simply put a simple test light in its place. The light will stay on bright until you unplug the "bad" part. I say that because it is possible there is a short in the wiring to the part and not a bad part.

Fuse 4 comes out of the fuse box as two pink wires, One goes directly to the Mass Air Flow sensor on the top of the air intake filter housing. Another goes to the EVAP purge valve that is on the top of the engine toward the middle of the manifold. It provides power and the ECM provides a ground to open that valve and the engine can then burn off the fuel vapors stored in the canister. The last one goes to the cylinder deactivation system which isn't used on your 6.2 engine.

The only part that will cause the reduced power when that fuse fails is the MAF.
First thing to do is to check the wiring harness where it runs from the MAF and the purge solenoid and to the rear of the engine. If it take a while for the fuse to blow it could be the wire is shorting out along there or at the unused connector at the rear of the engine.
Take a cheap incandescent test light (or even a simple 12 volt bulb and a couple wires) connect it in place of the fuse. Just turn the key on, not to start. That should apply power to fuse 4. If the light is on bright, unplug the connector to the EVAP solenoid first. If the light goes out or dims a lot, you have found the problem! If it does not unplug the MAF connector. If you unplug them both and the light stays on then you can go along the harness and move one small area at a time until the light goes out, in that area is the short.
Now you say that it took a bit for it to blow, not instantly? That would point more toward the EVAP solenoid and not the MAF sensor. That is because the MAF does not draw a lot of power and it stays more or less at a constant draw. The solenoid however does not open immediately, It does not operate until the ECM calls for it and it opens.

A very fast and dirty test would be to replace fuse 4 with a new fuse. Unplug the EVAP solenoid on the side of the intake and start it up. If it is a bad solenoid it should run and you could drive it a bit until it relearns the idle speed. If the fuse does not blow in the normal range when it has been blowing, you know it is the solenoid. It will probably turn on the check engine light for it as it will not be able to turn it on but that is a fast test. Then buy a replacement and install it. Once the system has run and the EVAP test runs okay the light will go out on its own.

First two pictures = MAF circuit and location. Second two are the EVAP solenoid and its location.