the axle seal leak is fairly common amongst all 97-up f150s.
if it is a small leak, let it ride unless it's just bugging you. as long as the seal doesn't completely let go you'll be able to ride on it for quite some time. check the gear oil every oil change and make sure the leak doesn't get worse.
it is always a good idea to change the seal whenever you get the time though. i rode on mine for a little over a year and it went on me 20 miles from home. i limped it home but ended up having to replace an axle. my axle tube still isn't right either.
2002 F150 FX4. Black two tone. Extended cab.
-SI/DO flowmaster catback
-AFE cold air
-EDGE Attitude. Level 2 Custom.
-Headmaster Shorties. One cat for each side.
-305/70R16 ProComp Xterrains-16" American Racing rims.