When I put it in 4x4 and accelerate up to like 50 it vibrates like crazy, then I let go of the gas and the vibration goes away. It's bad, like I didn't even want to take it that fast... but I want to sort these vibration problems out. Any input?

 

How about a front driveshaft, U-Joint, and/or bearings on either end of the driveshaft. It should be the only thing not turning if you are in 2wd.