JGroups has been around since 1998 and is therefore very stable, not dated at all ! It is being actively developed (I'm the lead dev).
It is used by a lot of applications/systems out there, if you google for it, you'll find a lot of references. Just this week, a telecom company using it broke the record for largest cluster built with JGroups with a 1115 node cluster !
JBoss (Wildfly) clustering (Infinispan) uses JGroups as well. I suggest write a small prototype and see how it performs. If you ask questions on the JGroups mailing list, the people there are very helpful...
I can't comment on Akka, as I haven't used it, but my distaste for Scala would keep me away from it anyway...