Are you getting less than normal fuel economy? Unless you've noticed poor MPG's as of late, I would suggest cleaning up the tip a bit with some steel wool or a scotch brite pad first and see how long it takes for the tip to turn black again.
At this point your vehicle is 15 years old and it may just be normal carbon build up on the exhaust tip. Of course if you are unsure of the age of your O2 sensors, it couldn't hurt to replace them anyway. FWIW, Rock Auto had the best price on them when I replaced mine this past winter.
His:'00 WJ, 4.7 Limited, stock
Daily Driver: '05 SRT4, Stock turbo, 252 WHP, 321 WTQ
Hers: '03 WJ, 4.7 V8, Kolak Exhaust, IRO 3", Bilsteins, JKS TB, KOR TB Bushings, 32" Cooper ATP's