Make sure the new tensioner pulley has the same amount of grooves as the other existing/stock pulleys. Also check that the belt has the correct amount of grooves. There are 6 and 7 groove pulleys and belts, depending on year.

 

It's your water pump. I shredded 2 belts before I realized this, then the pump seized.

If you haven't changed it in a while, it's almost a sure thing.

 

Had the same problem as you. Tried tensioner and idler pulley. Even checked the water pump and that felt ok. One weak later water pump pulley let go and almost came out of the pump. I had checked it not a day before. The bearing was wasted and was moving in the pump housing. The truck didn't leak until it let go.