I found the Grunge to be very different from the Rat 2 I used for several years. I heard the Grunge as being pretty scooped, compressed high gain. I heard the Rat as being rawer and having a broader range of gain levels, from “a little snarly” to “so distorted it’s edging into fuzz.” As I recall the Grunge was pretty dirty and sustainy no matter what you did. I don’t recall being able to approach a cleanish sound with it. On the other hand its high gain settings were pretty much supernatural sustain levels. Some might dig that.I didn't have the grunge pedal, but I had a DOD multi fx unit back then that had a grunge setting. It was fun.
There should be a ton of them out there though. It was one of the highest selling pedals at that time, even more so after Kurt put one on his board (apparently as a joke). Supposedly it's just a modified Rat.