Got my gf a used 400ex with pro taper bars and PC slip on and it resonates/vibrates you so bad that when you get off your hands are almost numb....anyone experienced this before?

Check your motor mounts or it is your crank.

as in a bad crank bearing or out of balance crank?

mine vibrates a little, but not to the point where it hurts or makes my hands numb.

The crank probably needs to be trued and balenced.better get that fixed befor it screws up your bearings

Out of balance crank is my guess,how and why it got like that is anyones guess. I had a raptor with the same problem,it did break a motor mount. Mine got like that when I had Trinity Racing build it up and they never trued the crank to my new piston and rod. It may not not seem like much but remember the revs the motor runs at.

With the crank vibrating,you'll lose some power.
What mods does it have internally(piston,portwork,cam)

stock except for silencer and pro taper bars

the bars will vibrate a lot more than stockers. the cross bar makes them super stiff, and you feel the vibrations a lot more. i noticed when i put new bars on mine i felt the same thing, but you get over it after a couple rides.

if it was a noise also, check the slip on, cuz the tab or whatever holds the thing on towards the back may have crcked(ive cracked two stock slip-ons, and my WB slip-on 3 times and every time it sounded like a lot of vibrating)