I don't think you are in fourth, you shift into third, then the torque converter locks ups (it's supposed to). With the torque converter locked, it is just like a manual transmission in gear. You can hit the speed limiter in third. When you hit the limiter, and you back off the gas, do you then go into fourth? If you turn OD off (The little button on the side of the shifter, light in dash "O/D off" Lights up) you will not go into fourth (OD) and remain in third. You can tell by RPM's in this condition if you are in third when you try your test in drive with OD light off and see if the RPM's are the same.
At 80MPH in third, I am at about 4200 RPM. At 110 , you should be up in the 6000's (in third).
If this is the case, there is nothing wrong.
The squealing is probably the serpentine belt slipping when the RPM's change on the shift.
Just in case I missed something, have you checked the color and smell and level of the transmission fluid?
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