It all started when I got into it a bit while in 4x4 and there was a clunk. After that anytime I went over 50km/h it would vibrate and make a bad noise. Then it went away one day when I had a mechanic in the truck, I went over 100km/h on the dry asphalt with no problems... Any ideas? I haven't been able to recreate it, but I'm nervous to depend on it and go up north for a week.

 

That figures...never happens when the mechanic is in the truck! I would definatley check all U-joint carefully. Thats first and easiest thing to check. Can you tell where the vibration is coming from...front end, rear end, hold your hand on the t-case shifter and feel vibration or grinding while it's locked in?