It should be noted that there are no episodes of TNG or DS9 that would give a craft designated "shuttlepod" or portrayed by the Type 15 or Type 18 miniatures (or the joint prop) warp capacity.
That is, while sometimes these craft travel "on their own", they are never credited with interstellar journeys, merely with assorted arrivals or departures; they are never stated to be at warp; they are never shown to be at warp; and the nacelles of Type 15 are actually explicated as impulse engines in a readable graphic in "Descent", verifying for a rare once the designers' intent.
We could deduce, then, that the difference between a shuttle and a shuttlepod lies in the latter's inability to accommodate warp engines. Impulse engines, yes. Subspace engines, very definitely (the "Descent" graphic and "Destiny" dialogue establish the pods to have the ability to generate subspace fields with their engines). But not warp engines.
It might be technologically flat out impossible to equip a lifepod with warp drive, then. Or then merely challenging and expensive and not worth the hassle. The interiors of Type 15 and Type 18 suggest that "impulse nacelles" or "impulse cowlings" are relatively standalone pieces of hardware, not taking up further room inside the hull with their subsystems or whatnot, so those could perhaps be bolted onto lifepods, too. But warp engines might be a different matter, requiring at least the underfloor or back compartment volumes supposedly available in the shuttlecraft. The difference in size isn't all that great, though, so most probably it's a matter of warp engines simply being way too "expensive" (in terms of materials or power costs or something like that) to be used willy-nilly on any small craft.