sometimes my jeep wont turn over so i tap on the starter and she takes right off. do you think i could just replace the selenoid rather then the whole starter?

 

Most likely, no.
The issue is with the starter brushes in most cases. When you tap on the starter, it makes a slightly better connection to the armature, and gets it spinning. Sometimes it stops on a dead spot.
Starters on an XJ are relatively inexpensive, and one of the easiest starters I've ever seen to change.

 

Tried to do that with a customer when i woked for advance... The aolenoid was more expensive then the starter