To replace the outer seals you have to replace the bearings.... The outer seals are only supposed to retain the grease that should be packed in to the bearing... If the inner seal does it's job & retains the gear oil the grease generally isn't a problem... Make sure to thoroughly clean the bearings & pack them with good quality bearing grease & unless you see leakage I wouldn't replace the outer seals...
Personally I use a grease needle to inject grease in the bearings...
