Does the 1991 Wrangler 4.0L have a self tensioner

Sounds like you have all the info you need but are afraid to try the job. Look under the P/S pump and you should see a bolt that runs under it (left to right). You don't loosen the pump pulley, but actually you loosen the pump itself. Loosening the bolt underneath will remove tension from the belt, tightening it will restore tension, but only after loosening the two bolts that allow the pump to slide back and forth on the mount bracket. Make sure you know where the belt goes before taking it off (draw a picture if you need to) and, remember it is very easy to overtighten it. after installing, belt should have about a half inch of deflection. Check this befrore starting and again after about an hour of running.