How do you engage 4WD in 2001 Wrangler?

Depends on if you have the electronic or manual transfer case. The electronic ones, they advertise as "shift on the fly" capable, but this is a really bad idea.

Either way, put the vehicle in neutral... if you have manual locking hubs, engage the parking brake and lock the hubs by rotating them clockwise to the "lock" position. Then, if you have the manual transfer case, you pull the transfer case lever either to the 4HI or 4LO position... if you have the electronic transfer case, push the button either for 4HI or 4LO.