Won't engage 4X4

[torque]

My '07 LBZ won't shift into 4X4 until it's throughly warmed up. When the engine and trans temps are normal, it shifts just fine.
Anybody have this problem and know what causes it?
Thanks

[More Power]

Are any related trouble codes being set?

[torque]

Sorry about the delay, I had to deal w/ a personal problem.
When I press a 4X4 button it blinks for an extended time and finally I get a message to service 4X4 drive - no code.