Read this on WJJeeps:

I didn't know this when I first got my Jeep and I'm pretty sure I topped it off with ATF+4. Am I screwed? I haven't had any issues at all and it's been about a year and a half. Do you all adhere to this pretty well? Any experience deviating from this? Where is the best place to buy MS5931?Mopar®P/N 04883077 Hydraulic fluid / Power steering fluid (MS5931)
MS5931 IS THE ONLY FLUID THAT CAN BE USED FOR THE POWER STEERING SYSTEM. NO EXCEPTIONS!
Important note:
The factory fill power steering fluid for all 1999-2004 Grand Cherokees is a unique fluid, MS5931. ATF+4 (MS9602) should not be mixed or used as a "topping off" fluid on the Grand Cherokee WJ. All WJ models should be serviced using ONLY the special MS5931 fluid. ATF+4 is NOT an approved Power Steering Fluid and should never be used under any circumstances. Note that all fluids will darken in color with usage and fluid color is no longer an indication of fluid condition.