What is this?

I noticed this hose is split and disconnected from it.

 

Vacuum line from the transfer case.

 

Transfer case shift actuator. Repair/replace the vacuum hose.

 

If I see the positioning correctly in your picture ; Transfer case actuator,

I would be replacing that hose with new ASAP

 

Its just a vent line. No vaccum. Just cut the end off where it is broken and reattach. If it's too short then you may have to replace but I'm not sure where it goes.
Edit a lot of people posted as I was typing. There is no vaccum to this. Every bit of 1st gen 4wd is electronically actuated AFAIK.

 

You can probably find a pneumatic fitting to extend the hose with a new line. Measure the ID to determine what you need.

 

Thanks y'all!

 

This is a very common inaccuracy ... As was stated prior, this is just a vent line. I've made it my life's journey to correct this misconception every time I see it...

 

Few months back when searching for info on these hoses, I ran across a thread here explaining these "breather hoses" the FSM refers to and their locations. My 04 DC 4wd breather hoses were indeed found in the locations mentioned in the below thread.

Breather line location guide | Toyota Tundra Forum (tundras.com)

.

 

I believe N84434 is correct. I don't think it really does anything. As long as you can keep moisture/dirt from entering the actuator, you should be fine. Cut the split end off and securely reattach.

 

Snipped off the split-end and reattached

Thanks again!

 

Still, if a replacement isn't immediately available, a piece of vacuum hose can be used to repair it. But, I congratulate you on achieving little life goals.