2000 Gmc 3/4 Ton 4x4 Engagement Problem

[sandrunner]

Have 2000 GMC 3/4 ton pickup 4x4 with 160K on it.

Last summer intermitent working of push button 4 wheel drive engagement and now this winter would not go into 4x4 at all.

Checked every poss problem and new encoder motor installed. Have signal to transfer case but the transfer will not shift into 4 whl dr??

Is there a fork or something inside the transfer case that can stick or fail. No grinding or indications ever of transfer problems it just will not shift into 4x4 mode.

Any thoughts as to what it could be and any fixes would be greatly recieved.

 

Thanks