I got this diff (see picture) and wondered if there was a seal missing. It's a push fit and when the shafts are out I can see right into the diff and hence any oil could come out. The diff has oil over it (as you can see) I just wonder if it is due to something missing or if any seals would have failed or even if it's just because it's been transported with the drive shafts out.
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I looked in the manual (Haynes covering 1982-1993 Sierras) and it has no mention of anything extra being fitted. I've attached a pic ot the drive shaft end too.

Sorry for pic quality (taken on mobile phone).
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Looks like the seal is still in place and most diffs have some "spillage" marks, especially when the shafts have been taken out.
Pat...

Mine is like that and it's been fine so I'm sure that's how they're meant to be!

Pete

Will it need more oil when I fit it?

I'd dump the oil anyway and refill it, for the sake of a couple of quids worth of oil!

Yeah I suppose it's not worth messing with for what oil it will take. Never thought of it like that!

Push the shafts in, give it a good clean with a stiff wire brush, dump the oil, but don't re-fill it till it's in the car, or it may just leak out again during installation etc.
It's easy to fill up - you just stop when it starts to trickle back out of the hole.
Pat...

All Fords have built in oil displacement,its to stop them rusting!

You may the filler bung is very tight, if you are unable to undo it weld a large nut on , that will shift it.