Just bought a Jeep with lean fuel mixture- 2000 stock cherokee
 

I just got a new Jeep and was informed that, when it was taken to the mechanic they made the fuel mixture lean for some reason and is now causing issues with accelerating and hesitating, and has stalled before. I'm wondering if anyone knows how to adjust the fuel mixture on a xj, from my understanding there is no adjustment screw so I'm at a loss and don't know what could have been done to my Jeep to make it run leaner! Really need y'all's help, thank ya kindly! P.s just had fuel pump and crankshaft sensor replaced

The mixture on your XJ is not adjustable. It is managed by the computer (PCM). The primary components involved are the MAP sensor, and the O2 sensors. These are responsible for measuring the air coming into the motor, as well as the amount of oxygen remaining in the exhaust gases after combustion. By monitoring the values these sensors provide, the computer can adjust the amount of fuel being delivered via the fuel injectors. Any competent independent mechanic should be able to troubleshoot a mixture problem and help get things back to normal. If you are doing it yourself, I recommend having access to at least a cheap OBD2 scanner that can provide you data on O2 sensor voltages and fuel trim values. This will help you better understand what the computer is doing, and give you a starting point. It can also help to show that whatever fix you decide on has been effective.

Failed MAP, O2 sensors, and air leaks (called vacuum leaks) are the primary culprits.