Your forensics are quite good, but you missed actuall change that caused your problem
the change that is affecting you is this https://github.com/wildfly/wildfly/commit/8510bc122868512aff7744ca027b3c9d5d08dd03
we switched json provider from jackson1 to jackson2, which is the provider & dependency that is not added by default.
Before this change jackson1 dependencies ware added to classpath.
Jackson2 is much better & faster than v1 ever was, I would recommend you to upgrade to jackson2 and all your problems should go away.
see http://wiki.fasterxml.com/JacksonUpgradeFrom19To20 on few hints on how to do upgrade.
if you cannot upgrade to v2, you can still add jboss-deployment-structure.xml and exclude new provider and add old one and your app will still work as before.
Yes, I saw that it changed to the resteasy-jackson2-provider, but it's still adding a dependency on jackson-core-asl (lines 64 and 86 in the commit you mentioned), so if I understand correctly, it's now loading both Jackson v1 (core only) and Jackson v2.
My war currently uses Spring Web MVC for REST stuff instead of RESTEasy. I tried adding entries to my jboss-deployment-structure.xml file to exclude the jaxrs subsystem and associated module, and it worked ok for WildFly 8.1.0.Final. I tried loading the same on JBoss 7.1.1.Final as well (what we're currently using) but it doesn't support the exclude-subsystems tag. Right now I've got the jboss-deployment-structure.xml set to exclude the jackson-core-asl and jackson-mapper-asl modules so it will use the versions in my war. Not saying it's the best solution, but it seems to work ok on both WildFly and JBoss AS.