I know this is quite old, but I'm replying just so others will get the correct info if/when they come across this thread in a search.
The belt tensioner on the 96-01 4.0 XJs is the idler pulley below the power steering pump. Just loosen the bolt that holds that pulley and then turn the adjustment bolt to tighten / loosen the belt.
The 87-95 XJs used the power steering pump as the tensioner, so you have to loosen several bolts on the pump and then turn the adjustment bolt under the power steering reservoir to tighten / loosen the belt.
No XJs came with the spring loaded belt tensioner that many people are used to seeing.
1992 Cherokee Laredo - all stock, DD. Needs some minor rust repair, then on to the mods!
1996 Cherokee Classic - 3.5" lift / 31.0x10.5 Mastercraft Courser CTs / Warn XD9000i - Stripped for parts / junked :(
2004 Grand Cherokee Laredo - 4.7, bone stock - Wife's DD
NJ F-Body Owners Association