The difference is which package versions are approved for use.
For the most part, you can add the yum configuration for CentOS or Fedora to RHEL machines, and install newer packages without breaking it.
Note that some packages will end up pulling in a ton of other dependent packages. By adding a Fedora repo to a RHEL machine, the following things will happen: 1. You will likely lose official support from RHEL. Anything that goes wrong is your fault. 2. Your OS install will start becoming Fedora; as you install more and more stuff. 3. Your setup could break entirely because RHEL packages are not tested to be compatible with Fedora ones. The core configuration is different in places, and generally you should not start updating the core components as this will break things.
My recommendation is to setup the CentOS/Fedora yum repos on your RHEL box. Run yum install for new PostgreSQL and see what yum wants to install. Eyeball the results intelligently and take some guesses as to whether it will break things. Cross your fingers and approve it, then disable the CentOS/Fedora repos you added. Carry on with your life and don't tell IT what you did.
If you can't connect your RHEL machine to outside network, repeat the process somewhere else to figure out what RPMs are needed. Download them elsewhere, and stick them on the RHEL machine and install manually.
As an additional note; I have setup a Fedora repo on a RHEL machine and changed the flag in the system to change the distribution ( there is a core package everything depends on... ). We then updated all, and the resulting system still functioned. Very evil, but it didn't explode. Also; it took quite a while to install all the packages. It essentially is converting the whole system from RHEL to Fedora in place...