Unless the motor was switched, a 91 didnt come with an HO motor in it.
Did you unhook the battery cable before you worked on the starter? 1 wrongly placed spark i.e. positive cable touching a ground and arcing, will destroy the brain of the ignition....ICM.
Did you check the spark? is it a weak yellow at a plug or bright blue? The ICM will allow the system to cycle with a weak spark, but it wont fire. Most you will get is it will sound like it wants to start, but dies as soon as you let off the key.
You have fuel, it rolls over, and you have spark, it has to run. Those are the basics.
The ICM is an aluminum looking box that is mounted to the inside of the driver's side inner fender, down in behind the windshield washer bottle. Unfortunately, i dont think there is any way of testing it, other than to try one out of a friends jeep and see if yours starts...they are between 60-80 bucks to go get a new one.
I just put one in my 87 4.2L and it was 74.99 at NAPA.