1999 grand Cherokee jeep transmission slips?

transmissions are tricky, this problem could be as simple as a missing or loose part, to a bad transmission alltogether. But either way, it will most likely have to come out and be fixed, rebuilt, or replaced. The only way to relly know is to take it to a specialist, theres no way around that. and changing the trans fluid will not help but it may have the wrong fluid in it also which can make it slip. im only a level 1 auto. tech. but i hope this helps you