2010 sequoia 4x4 won't engage

New member here. When I turn the 4x4 knob from 2H to 4H there is no response and the 4x4 light flashes steady. I cannot turn the front driveshaft by hand. Can anyone give me an idea on where to start troubleshooting? Thanks so much.
Update. Got both hands on the front driveshaft and was able to turn it. It does spin freely. Both 4Hi and diff lock lights flash steady when switch is turned, both reset when engine is turned off and back on. Diff lock button has no effect.

 

Hey hopefully you have fixed this ? I had a similar problem with my 2012 sequoia , my transmission ground strap was falling apart & was the culprit.

 

That is something I did not look at. I'll check it. Thanks

 

Hopefully thats it . Other more expensive culprits : 4wd computer & transfer case actuator which is around $1600 for oem & you have to drop the xfer case and crack it open to replace . I was going to just buy a salvage transfer case and replace the whole unit . Also one commenter on Toyota nation said that his agm battery was getting old and giving poor voltage , he replaced his battery and was good to go…..