I just got a 5lug axle from a ranger, and it has a rubber O-ring where the c-clip slides into. My old 4lug axles do not have these O-rings (i have never used this rear end before, i bought it bare and rebuilt it). Should i buy new ones or not use them at all?
Yes, you should have them on there. You can reuse the ones on the axles if you want, or you can by new ones. Sorry, I'm not sure what size the are, but I can try to find out.
They are there to provide a positive fit for the C-clips so they don't fall out during assembly. You can still get them from a Ford dealership the last time I checked.
Although there is a lot of torsion going on within the rear end under load, they may also serve to help keep the C-clips in place during driving...but that is just speculation on my part.
If they are there, leave them there, they will be fine. They tend to dry, crack and fall off, which is why you may not have seen them on your old axles.
I have axles with and without the o rings. I prefer to leave them out, because then you can pull the c clips easier, with just a magnet. They might cut down the end play on the axles, but I have not had any problem without them.