<font color="yellow">Sorry dude, but don't the rest of you guys find the image of a guy sprawled out underneath his Toyota, with a can of wheel bearing grease, stuffing gobs of it into the center-section of his differential... A little bit funny <IMG SRC="smilies/question.gif" border="0">"Repacking front diff"
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Seriously, if you haven't extended the vents on top of your axles with a new hose & fitting, drain & replace the gear oil in your axles every time you go through water deeper than them. The wheelbearings should probably be repacked about once a year, or about every 20-30,000 miles on a street-only truck, so use your judgement from there based on how hard you use the truck. I do mine about every 6 months, depending on how often I off-road it. It also depends on what kind of shape your seals are in - since newer seals keep mud & water out better.
The front locking hubs are a different matter from the wheel bearing. If your hubs are VERY hard to turn, they probably need relubed now. From my experience they tend to be most susceptible to water and mud, and need redone at LEAST every 6 months. More like 3 if you get into lots of mud.
Also, remember that unless you've done something about it, anytime you get water to the interior level of your truck, it also rushes into your transmission and transfer case...
The maintainence information I'm giving here is based on mud and river crossing type off-roading like we do down south, not rockcrawling.</font>