Well, the land speed business was just because they're a mass of dangling organs without appendages. What about a compromise for a 20 ft land speed "naked" and 30 ft with a skeleton? It just makes more sense to me that they could get around better with legs.

We also need to add the possibility of a bite attack and increase the claws to 1d6 (per the original). And add a blurb about putting on and taking off the skeleton. Here's a go at it:
Wear Skeleton (Ex): A visceraith can inhabit the skeleton of a humanoid or monstrous humanoid creature of its own size. While wearing a skeleton, a visceraith gains a +5 natural armour, a 10 ft increase in land speed, and two claw attacks (doing 1d6 damage for a Medium-sized visceraith). A skeleton-wearing visceraith can use the skeleton to wield weapons and perform other physical actions. A visceraith requires 1 minute to don a skeleton but only a full round action to remove itself.
Some visceraiths wear alternative skeletons. For example, a visceraith that uses a skull from a creature with a natural bite attack gains a bite attack (1d8 damage?).