When I break at over 40 mphs my steering wheel shakes, and there is a knocking sound that appears to come from my front drivers side wheel. Any idea what this is an indication of?

Thanks for your help!

 

"usually" warped rotors...
knocking sound, take your pick:
tie rod
ball joint
wheel bearing
control arm bushing
strut bearing
and a few more..

 

unbalanced tires (to add another)

 

Well rotors are cheap enough. I can replace that myself. I am hoping it is something that I can fix myself without taking it to a mechanic. Hopefully it's just a warped rotor and not one of the many other things.

 

balance problem will show up all the time..not just when hitting the brakes.

 

yep I would say blown boot and warped rotors

 

I would change the pads too, if it was me.