when putting Jeep in drive (1999 6cyl-auto) slips

Have someone, a shop etc, check your shift selector switch. It may be getting worn or just need some adjustment. Also be sure to check the fluid level, when the Jeep is in Park and the engine is idling to make sure that the fluid level is not too low, as this can cause your symptom as well. When was the last time you had your Transmission serviced because a dirty fluid and/or a dirty filter can also cause this type of problem?