Plz Help Me Fix My 2001 Dodge Durango
 

I have a 2001 Dodge Durango Slt Plus 4.7l and well for the 1st issue the brakes booster and master cylinder was replaced by myself and my father and now the brakes sometimes cuts out idk why. 2nd problem when i bought it the owner before me said the blower motor was bad but air comes out of the vents like on low setting or something but i looked at the resister and they already put in a new one so idk whats with that. 3rd problem 4wd wont engage the light just flashes and then switched back to 2wd. 4th problem is that i saw that they disconnected the ground from the negative terminal on the battery to the frame would that bother engine performance or make it run weird? Thx for your time. :)

I'm assuming you've checked the fuses/relays related to the blower motor. Then I would check to see if power is getting to the resistor and motor itself. The air you're feeling could just be the air that is naturally flowing through the blower assembly when you're driving down the road. It doesn't necessarily mean the motor is being powered.
I've had my 4x4 do that before because one of the plugs under the truck for the transfer case was ripped out by a snowbank. It may also happen if the shift motor was stuck. Hopefully it's nothing internal on the transfer case. Can you hear any noises when you try to shift it? I would assume there some kind of a 4x4 module somewhere too but that's purely just an assumption.
I doubt the grounding issue you describe matters in your case but it doesn't hurt to hook it back up and see what happens!