Make sure you are stopped, and in neutral before you turn the selector. If you are lucky enough to get it to engage once...keep shifting in and out of 4lo to free up the mechanism.
I cycle mine about once every 3 months. I run the 4hi every month for 10-20 miles. There is a post here where a guy split the Tcase. He included pics. It is an involved job. Not fun lifting the case in and out either.