Tensioner can't be adjusted , however I have had great success fixing them pretty easily . Remove it . Pin it to a vice , hit the spring area with PB blaster , work it back and forth 25+ times . Lube it up with fluid film , wipe it down , re-install it , be sure all pulleys are clean , done . Did this on my Black 5.9 3 or more years ago , not a squeak or squeal since . Did it to my 95 5 years ago . Just did it to my White 5.9 when I did the waterpump . Had to do it to a jeep I flipped . Worked every time .
If you don't have a vice , leave it on the jeep . Do the same as said above . Never did it this way but should have much the same effects .
I think the corrosion builds up where it pivots and stops the tension in the spring from doing it's job . It really helps bring life back to what seems a worn out part .
Did you get the right belt ? That's more than I can say right now . If you have the brand name and part number , then I would know for sure