Previously I replaced the idler pulley on the '01 TJ 4.0L and kept the factory one to rebuild.
Not sure how many members have done this, but I decided to replace the bearing and keep the rebuilt pulley as a spare.
Remove the long bolt and fender washer that holds the spacer to the pulley which then allows you to slide the spacer out of the pulley bearing. Then use a 30 mm or equivalent standard socket to press out the bearing with a vise. I chose 30 mm since it will allow clearance in the hole that the bearing is pressed into.
The bearing is a NTN 6203LU with a double seal made in Japan, but I found out that all the bearings that I could locate only sealed the bearing on one side instead of both sides.
I purchased a NTN 6203LLBC3 (PN 6L025) at Grainger for $7.15. I used scotch brite to clean up the surface that the bearing would be pressed into. Used the 30 mm socket and vise to press the bearing in and then reattach the bolt, fender washer and spacer.