How long have you had the problem? When you remove the shaft you will spill some gear oil in the axle tube. Its possible that the oil you're seeing is just some left over from the axle shaft swap. If its a lot of fluid coming out or is persistent leaking then I'd have the seals replaced. The seals can be damaged when re-installing the new shaft. To replace the seals you have to almost completely tear down the axle, removing the shafts and carrier.
2000 WJ 4.5L - Stroked, Locked, & Long-Armed
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WJ Chromoly Axle shafts w/Spicer u-joints, Lockrite p/n 2210, Nitro 4.56 D30 Gears