Had the same problem when I changed the fluid on my wife's WJ. Here's what I did:
Get a drill bit that fits in the center of the rounded out plug (3/8 IIRC), coat it in grease the drill the center out. The grease will help prevent metal shavings from falling into the t-case.
Took the largest easy out that I could fit and removed it using an open ended wrench.
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