01 Limited 4.7 V8, knocking noise=replace engine?

2001 Jeep Grand Cherokee limited, 4.7 V8, 4WD, quadratrac, about 105K.

My wife took our jeep on vacation, so I'm unfortunately not with it to give detailed info.
Anyway, as they were driving on the freeway, the check gauges light came on, and the engine started making a "metallic bang" sound, about once a second, and their was some vibration. It took them a bit to come to a safe spot to stop, and by that time, the frequency of the "bang" had increased (to maybe 5 times per second).

They towed it to a dealership, who started it, and it shook pretty good, although they said that as oil pressure came up, the shaking went down, but did not stop. Mechanic says bearings (unclear which ones) are shot, and engine will need to be replaced ($9000) He did not open it up at all, just started it. He said the crankshaft was likely damaged.

Oil levels were reported as fine, both by my wife and the mechanic. Mechanic reported oil as clean
Oil was just changed a few hundred miles ago.
No engine problems in the past with this vehicle.
No check engine light
No OBD codes. (At least from the on-off odometer routine)

The mechanic agreed to drop the oil pan and look inside to see what was wrong. I feel he was awful quick to say "replace engine", and well, the only people there were two women who know zilch about cars.

Also, apparently simultaneous with this, the battery was dead. They had to put it on a charger to start it. Unsure what this has to do with anything.

Thanks in advance for any and all second opinions.