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Could be the vacuum lines have a leak or maybe a stuck shift solenoid...I'm assuming you have auto hubs. So try turning the blower on high through your dash vents then engage the 4x4 if the blower switches to your defrost vents then you have a vacuum leak.
Is your 4x4 light coming on at all? It could be your Wiring at the shift solenoid. Sorry I can't be of more help. I have an 03 but mines been converted to manual hubs.
Is it actually not engaging, as in you hear the T-case servo go, front drive shaft locks in, but the hubs aren't locking in?
Or is the t-case not doing anything?
Basically you need to do some more diagnosis, jack the truck up (all 4 wheels) and support on jack stands, and see where the issue is. It could be electrical, it could be the servo at the t-case, it could be a bad t-case chain. It could be a problem in the front diff. It could be bad lockout hubs.
Just telling us that it won't engage doesn't give us a lot to go off of.
Is it actually not engaging, as in you hear the T-case servo go, front drive shaft locks in, but the hubs aren't locking in?
Or is the t-case not doing anything?
Basically you need to do some more diagnosis, jack the truck up (all 4 wheels) and support on jack stands, and see where the issue is. It could be electrical, it could be the servo at the t-case, it could be a bad t-case chain. It could be a problem in the front diff. It could be bad lockout hubs.
Just telling us that it won't engage doesn't give us a lot to go off of.
Generally speaking, if these trucks are kept in decent repair then the most common problem is a loss of vacuum (presuming stock lockouts). This can be caused by the diaphragm inside the lockouts rupturing (not unheard of if the hubs aren't lubed and cycled roughly every year), or by a broken vacuum line (again, not unheard of for a 7 year old vehicle). If it's a diesel truck then it's possible the vacuum pump has died since diesels don't create their own vacuum. If it's a gasser then this is not the problem. If this is the problem then your friend can either repair the vacuum leak by replacing parts, or plug the leak and replace the lockouts with true manual lockouts.
If it's NOT vacuum related, then he'll have some serious work to do because something is actually broken, or it's electrical which can be difficult to track down if it isn't a servo or switch - wiring is a PITA.
Either way, it sounds like he needs to do some more diagnosis to see what exactly is going on. Hopefully some of this information can be helpful once he's determined where the problem is.