So much problems with my '96 GT

First of all, when I first start the car and go into the back and listen I hear this high pitched "eeee" sound coming from the back, I was told this is normal. So okay.. I'm driving I shift into 4th gear at 42 MPH and now when I press on the gas I hear this "hmmmmmmmmmmm" sound... when I let go of the gas, it goes away... Then I press on it again, I hear it... This only happens in 4th gear. What is this problem? I recently took my car in and they kept it for a month and said they re-builded the entire back "differential gears" or whatever, Yet I still hear the problem? What could this be.

Then all of a sudden, I'm driving and I see "ABS" on the dash board so I'm thinking, ah this shouldn't be no biggie. So I read the manual and it says "WARNING: IF LIGHT STAYS ON MORE THEN 5 SECONDS HAVE A TECHNITION CHECK IT IMMEDIATLEY" so damn.. I don't know what I could've done for the Anti Lock Brake System to be disabled. What exactly is going on this this car? Should I just sell it and get a Cobra SVT (which is what I originally wanted to buy) I need some opinions here...

I'll check the fluid level to see. The noise happens in 4th and 5th gear.. but kinda stops after I'm going about 60 MPH, but from 40 MPH - 60 MPH it has the noise when I step on the gas.

Is it true around the late 90 models the sensors for ABS is malfunctioned after a certain point? This is what my friend is telling me, "just a bug that the manufactures didnt fix" and just wondering.