US Navy's E-6B upgrade nears production
 

The US Navy is putting its E-6B C3 aircraft through an upgrade and service-life extension programme that should see them remain operational until at least 2040.

http://www.janes.com/news/defence/naval/

Which begs the question... What happened to the AF/E-18G Growler?

Two different roles. The Growler is a jamming aircraft while the Mercury is a command and control aircraft. Mercury's mission (as Looking Glass) is to act as a command post in the event that the normal command post in Offutt Nebraska is taken out in a nuclear war. In a nuclear war orders would come from the NCA (The president on NEACP a USAF E-4) to Looking Glass and then on to our Strategic Assets (Boomers or Air Force bombers and silos)

It also can control more conventional forces if necessary.

EDIT: Were you thinking of a EA-6B Prowler?

yep ... That is what I was thinking about.