What size are your rocks? Are you trying to pry them out or are you just wanting to move them.
A great source for auxillary hydraulics is picking up an old lift gate assembly. I've had great luck with them as a starting point. You have the twelve volt hydraulic pump and a cylinder. You just need a place to stand and you're Atlas.
I have access to three, yup three, wheel chair lifts out of commercial buses. They have the built in hydraulic pump, cylinder, a screw drive assembly like in a gate operator, and some really neat portable controls.
Sooooo many projects, so little time.
If you're liking the trailer idea keep in mind that Northern carries axle stubs down to two thousand pounds with hubs, tires and wheels. For about a hundred and fifty dollars or so you can have some squatty tires with hub assemblies you can weld up to any old box you dream up.