2003 Ford Explorer xlt has humming and stuttering in front

I just bought a used 2003 ford explorer xlt with 95,979 the next day there was a stuttering when pressing on gas and a humming noise coming from the front. I had to repair upper ball joints and outer tie rods but the humming noise is still there and the stuttering has something to do with the 4x4 in the front needs to be replaced, it goes across the front that was all I got from the mechanic. I got minimal information because they just want the work. I need to know the parts needed for that and the price if you know.