What's the best way to look and or find debri?...
Inside the transom is a cleanout/inspection cover. if this is not accessible, then look up through the rock grate/intake for debris.
Take a close look at the U-joints for a vibration, too. Wrong flywheel/flexplate and/or balancer can produce a vibration, too.
The WJ pump has a plastic wear ring. If it has EVER been run on the hose, it needs a new one. This, along with shimming the front bearing will help if not eliminate the 'slipping'. Poor impeller condition will make it slip (cavatate) too.
Dan'l