I will give you some advice, but it comes from changing the seals in my toyota, not the ranger.
Pull the wheel off, unbolt the drum from the axle, and slide it out. Jack the truck up high on the side you are working, so that all the grear oil stays in. Get the old seal out, and pound a new one back in. A good strategy is to freeze the new seals, and they shrink a little bit, otherwise it can be a nightmare. Buy a couple extra, just in case. I ruined 2 before I got it right.
Also, where has the fluid gotten to? If it is leaking before the drum, then just the seal is bad, if it has gotten into the drum, then it leaked past the seal AND the bearing. If rangers have "sealed" bearings, then that one is no longer sealed, and should be replaced while everything is apart.
Once again, I replaced these seals on my toyota, but have NO idea if it is the same on the ranger, I'm sure someone will follow this up though.
2003 2wd 2.3 std XL