Troubleshoot 1999 Cadillac Escalade engine that starts, but runs as if it will explode?

Thanks for responding rasmataz;I just got back into town. Here is what I know.
My son had the vehicle towed home when the vehicle didn't sound right and he saw what he thought was motor oil leaking from under the vehicle. The next morning I checked the oil; full, black, and didn't smell burnt. Got under the vehicle and determined the fluid must have been from the differential. Upon starting the engine I immediately turned it off. The whole vehicle shook and I thought it may be a piston about to let loose. A friends ex-husband was in town listened to it and said "top not bottom, end; bent valve is my guess". Having just recently purchased this vehicle I'm low on funds, so I can't afford to bring it to a shop. What, should I do next?

Hello rasmataz; no compression @ #3 & rusty brown plug led us to. After removing the head we saw swirling scars on piston surface, no debris evident, and no resistance. Valves appear fine, as do all remaining components of the top end. Conclusion: piston not connected > spun bearing or broken rod. We'll pull the engine & disassemble bottom end tomorrow, if all goes well. Your input on what we have done, concluded, and are planning will be both appreciated and valued. Thanks again.
Swizzle1 & Son