I have an 86 jeep cherokee 2.5l 5 speed manual, I was driving I started hearing a terrible knocking noise, it knocks in first, second, third and fifth... Fourth gear is the only gear that it doesn't knock, any idea of what my problem may be..?

 

Sounds to me like the cluster gear is the one that may be involved with the noise. Does it make the noise in reverse ? 4th gear doesn't load the cluster usually, since it's direct through, but all the others run power through the cluster gear. ( I think ! )

 

Yes even reverse makes the knocking... Fourth is the only gear where it doesn't make a noise

 

Maybe there is a tooth chipped or partly broke off on the cluster gear, which could make the noise you hear....(?)
You could drain the transmission, & see what metal may come out with the oil. ....

 

Yup. 4th is the only "direct" gear in the trans.

 

How hard is it to replace the cluster gear and is it even worth it to replace?

 

Hard, and not worth it. Find a good used trans.

 

X2 on swapping it out.
The old three-speeds were bad enough to mess with, but 4 & 5 speeds have a lot more parts in them, so you don't wanna go there...

 

What tranny am I looking for..? The ax15? Ax5? Just so I know when I call around and will a 4.0l tranny work if I use my bell housing and transfer case?