What seed-dispersal method would such a plant be likely to use?
For the sheep-fruit? none.
The umbilical will simply snap & the 'sheep' drop off the plant right there & walk away.
For it's seeds to propagate itself? the sheep-fruits' legs, they 'are' the dispersal method.
The sheep-fruit would just wander off grazing as sheep are wont to do.
Seeds of the plant are grown in the sheep-fruit & deposited with its dung as it wanders & grazes.
It's likely the sheep-fruit don't propagate 'themselves' directly but only indirectly through propagation of the plant species they fruited from, they'll be part of the life cycle of the plant & vice-versa, they will be it's seed dispersal method, the seeds grow in the sheep-fruits womb-analogue & are 'birthed' as it poops to insure a little parcel of fertiliser for the seeds, all the sheep-fruit will be ewes, there will be no rams, if the sheep fruit aren't integral to the plants actual means of propagation & seed dispersal then however they came to be (genetic engineering?) they're just dead weight evolutionarily speaking that will be heavily selected against & eventually disappear without constant intervention.
To keep them looking like ordinary sheep constant intervention by selective breeding will probably be required, harvesting (culling?) of immature sheep-fruit with any variation from the norm while still on the vine or otherwise immature (unripe?), before they're reproductively mature, is probably needed.