99 Blazer Vacuum lines?
 

To make a long story short, can the TTCM pick up a code for a broken hose? Sometimes the 4x4 works and others its a no go. It will usally go into 4lo no problem but when you hit 2hi it goes into 4hi but the 2hi button is on. Once you start driving there will be a very loud cluck and it will disenage into 2hi.

Not typically, but if it doesn't see the front axle engage via the little electric switch on the front axle, it may abort the shift.