you can pop a seal off, flush out with wd 40, re apply the seal after regreasing.
TeamFastEddy.com sells a unit called the Greaser. its a bearing greaser that you can use with the seals inplace. its really quite a nice lil unit and can do many sizes of bearing used on most all our RCs with exception to some of the largest bearings found in the HPI Baja 5B diff (Big Bearings).