Hi azido, wellcome to the Elastix forum!
res_jabber.so is not compiled by default, you need to run 'make menuselect' before compilation and manually select it.
Here is a link that might help you:
michigantelephone.wordpress.com/2010/09/...terisk-installation/
You need to download the Asterisk srpm from Elastix, make sure it is the exact version you are running, make menuselect to select that channel driver and compile it.
Well, actually you can also download the sources from Digium and compile, but then you will not have all Elastix's specific patches, but they might not be needed by you, most of them just add some hardware support.
You can try to compile it in a different machine, and then try to copy res_jabber.so to your modules directory, this might also work, but I'm not sure.