I am going to rotate the tires this week to troubleshoot, but I was wondering what symptoms I would see if the half shaft was going/bad. I have replaced the wheel bearings, lower ball joints and tie-rod ends about 4 months ago and I am getting a constant roar on the highway....which has slowly been getting worse. I also just noticed that there is a high pitched squeal (like metal on metal) at random when driving over the last week or so. When it happens, it does it until I stop the car, then it goes away for a bit, but comes back. And speed does not matter. It happens at 15 or 65. It does not constantly do it though.

Any ideas????

Cracks or tears in the boot can lead to CV joint failure, as can normal wear and tear. Signs of worn or damaged CV joints include lubricant leaks, clicking sounds during turning, clunks during acceleration or abrupt deceleration and frontend vibrations.

Would those front end vibrations be constant at all speeds or would they vary? I would assume the steering wheel should wobble too, right? How loud would the clicking be?

very loud clicking.

A good test for worn CV joints is to accelerate (from a stop or very slow speed) while turning fairly sharply. This is where you will hear the clicking when the joint is worn. Figure eights in a vacant parking lot are good but making a right angle turn on a hill from a stop is even better because it puts more load on the joint.