The engines rear freeze plug will leak more with the engine running and with higher rpm. Since it is leaking sitting still (didn't say whether engine is on or not) can you get a mirrow to see where the leak is? If engine is off then Y pipe may be the cause. But the drive bellows could also be it. You said inspected but not replaced. After 5 years they become hard and develop small cracks. I am betting it's the bellows as the y pipe would need to have been hit or a defect in it to leak, low probability.
Also when moving a void is created at the back of the boat lowering water pressure on the bellows while the y pipe will have exhaust and hot water in it still.
1997 2400 SCR
5.7 Vortec / Bravo 2