I was cleaning up my 23 spline t-case yesterday when I noticed what looked like cracks, they don't seem to be deep and they feel like ridges not cracks when you run your finger across them. Do I need to worry about this?

 

Looks like age craking to me. Alot of old aluminum that has high pressure on it all the time has these kind of cracks. They don't look serious but i nthe long run could be. I'd just keep driving it unless you wanna change it? Go to a junk yard and get one.

 

I have another forward shift t-case and as I understand it the only difference is the input gear, if that is right I could just swap the 23 spline input into the 21 spline case and use that case?Would that work?

 

those lines come from casting and they are not cracks you see it a lot on aluminum to find out if they are actually cracks you could use the dye and black light.

 

That makes a lot more sense because I took a closer look and can see that they are small ridges, not cracks.
Is it better to use a gasket to join the transfer case to transmission or do you guys just use permatex?