A#1

i take it that by propeller gear...you mean the ring and pinion gear in the axle differential...if so...yeah 20,000 miles is quite soon for this type of maintenance...unless you do a lot of serious off road driving in deep mud puddles...sometimes with weak seals water can get into the axle...that's not good...to keep an eye on the grease level...remove the plug near the top of the axle...put your pinkie finger in the bolt hole and see if you can dip into the gear lube...if not top it off...if you can and it looks frothy or foamy...means water has gotten into the axle...drain the lube from the bottom drain bolt and replace with the proper axle gear grease...usually a 90 weight oil...so thick enough hence the name axle grease