First of all, you need to decide what year your Grand Cherokee is. 98/99 is wrong. It's either one
or the other, 98 and 99 are totally different models, one is a ZJ, the other a WJ.
This is a WJ (99-04)

This is a ZJ (93-98)

You probably have a ZJ if you think it could be a 98. The ZJ has an NV249 as standard.
Apparently the NV242 used in the XJ was an option on the ZJ (at least in the USA).
Not straightforward to fit one, though it can be done. There is a guide here
http://jeephorizons.com/tech/tcase.html