1999 was the model refresh from the flat-nose chrome front bumper to the "fat-lip" plastic monstrosity. A 1998 front bumper will be completely different from your 2001 ~ it can be made to work, but it's completely different top and bottom (and inside, the bumper support is different too).
To the best of my understanding, if you have the black SR5 flares (and not just the chrome sticky trim), you need a bumper from a 1998 Limited that has the correct holes for mounting the fender flares. That's true for the front or the rear, there are holes necessary to the mounting of the fender flares.
I've looked into this myself because the 1999+ front bumpers make me want to gag. No one wants to feel like hurling chunks whenever they look at their vehicle, so I'm going to retrofit the older chrome bumper. You can see the holes I'm talking about in this ebay listing:
Front Bumper Chrome Steel Primed Valance For 96-98 Toyota 4Runner Limited Models | eBay
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'99 Limited 4x4, Millenium Silver ~
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