MC-1 outdrive noise

1980 18' Fiberform Runabout
Mercruiser 470 with a MC-1 outdrive.
While cruising on smooth water at about 30mph, we suddenly lost power. The engine was still running strong, but the back end was making a clunking noise and acted like it was popping in and out of gear. Limped it back 20+ miles at 7mph to the dock. It would make no noise at low power. It shifted in and out of both forward and reverse with no problem, but would make the noise under higher load. I figured it was the U-joints. I've pulled the drive off and have found the U-joints to be solid. The gimbal bearing feels good too. Any thoughts?

Much appreciated.
Thanks,
Pat

check the drive oil level and inspect the gears in the upper unit

That's where I was going next. It doesn't feel rough when rotating it by hand. The castle nut is tight, so the backlash should be good unless it's very worn.

Ayuh,..... Drain the drive's oil, 'n look for metal,.......

Split the drive, 'n see if the lower unit input shaft broke at the o-ring groove,.....

Pop the cap off the upper unit, 'n check the gears for damage,.....