Random starter issue ?

06 4x4 4.0 Manual Trans.
Starter will turn, but not engage flywheel (at least not fully), makes that nice whirring grindy sound.
Happens completely random.
I can rock the truck back and forth in 2nd gear and it will then start.
I put new contacts and plunger in and problem continues
Replaced with a used starter and problem continues.
Only thing I can think of is chipped or missing flywheel teeth - flywheel was replaced 2 years ago with a new Toyota one.

Any other ideas of what it may be?

 

I agree. Sounds like some chipped or missing teeth on the flywheel. Is there an inspection plate you can remove and peek at the flywheel to confirm?

 

No inspection plate that I am aware of.
Might have to pull the starter and use a mirror or remote camera to see from there.

 

Usually, the flywheel is made of a stronger material than the starter so it won't destroy the flywheel.

 

Does your tranny where mounts to the engine block have this metal plate. It can actually be removed and look at the flywheel.

 

I'll look again.
That diagram is showing the starter on the wrong side though, so it may not be correct.