9 out of 10 times that will not work. when the jeep gets pluged into the machine is checks to see if the system is working properly, or if the system is ready. if you disconnect the battery right before you go you will get a "readyness code" which you will fail for. 96 to 2000 your allowed 2 readyness codes and for 2001 and newer your alowed only 1. But your kinda lucky since your jeep is a 91 it doesnt even get pluged into the machine. you just get a dyno test, where they get a rpm reading and a exhaust gas test. which proably wont pass anyway. there is a reason that the check engine light is on, fix it!!!! If you cant afford to fix it dont drive it!!!
pete