I have a JD510, it's old just like I am, and we are both worn out. Yes, my 510's boom and swing drift or sag when parked. This is because fluid is bypassing worn cylinders or control valve.
The relief valve should only function when the control valve is operating and sending oil to a given cyclinder. Once the control valve closes for that cylinder, that circuit should be locked. However, if the packings in the cylinders are worn, or the cylinders are deeply scratched, or the control valve is worn out, then this could causes this drifting.
JD has all sorts of complicated gadgets in their hydraulic systems that really take some head scratching to figure out. So if your boom, stabilizers do not sag or drift when the unit is parked, and only do so when digging, then you may need a JD technician.
This stuff is very expensive. My 510 has a 16 cylinder radial hydraulic pump on it, it trashed itself last year and a new one costs over $5,000, but a outfit called Kin-Tec rebuilt it for $900.