When I have my truck in 4 hi at about 35mph it starts to vibrate alot what could be the cause of this?

I thought it might be U-joints.

 

If you only experience the problem in 4 Hi it leads me to believe that, one, the front driveline is bent or as you stated, the u-joints are shot. To test it, mark the drivelines position in the transfercase and front differential yokes, remove it, place your truck in 4H and test it. If the vibration is gone then you have narrowed it down and you don't waste time and money replacing parts. Maybe this helps!