i just got a 1996 gmc sierra 2500 4x4. i went to go pick up a snow plow and my snowblower
,i put in the box as i started to drive my truck started to shake in the front end only when i got up to about 40mph it feels like a warped rotor the feeling. i got out and it look liked my parking was on cause i left a skid mark for about 50 feet. when i looked underneath i could only see my transfer case is leaking please help my stuck!!!!!!!!!!!

 

You left 4wd on while going down the road? If that's the case I'd say your transfer case is shot.