2012 f-150 4by4 issues, please help

[bluebombersfan]

I have been having a very annoying problem with the 4by4. It all started with my brakes. I was hearing a noise that my mechanic told me to install new brakes and rotors. He told me the rotors were warped and needed new ones because they were to far gone to machine. I spent $1000 for new pads and rotors on all for wheels. I get my truck back and suddenly now i hear a grinding noise coming from the front wheels. They tell me it's a problem with the 4by4 trying to engage. They replace the part from the driver side. I don't know what its called. It was a suction cup thing with a gear attatched. they said there was a leak and causing the the 4by4 to ingage. Paid another $500 for that to be replaced. Got my truck back and as soon as I drove it home, same thing. Then I got the other side done. Another $500. Drove it home. it was good for almost a week, then same thing again. dropped it off again today and now he said it might be issue with the vacuum. I have spent $2000 in the last 2 weeks on my truck and feel like nothing is improving. So now i am waiting for them to check the vacuum as I left it at the shop today. I am getting so stressed with the situation.
Does anybody know whats going on with my truck?
please help, I am starting to hate this truck.
Also I should mention the grinding sound coming from the front usually happens under acceleration past 40km/hour. I never hear it at low speeds or when I am coasting.

 

[85_Ranger4x4]

Dad's '05 had the alternator going out, it wasn't running enough voltage to hold the truck into 2wd so the hubs would drag.

The default to 4wd so that is naturally where they want to go.

100+ years and we are still struggling to make good automatic hubs.