"WSAD" has actually had three different names. It started life as VAJ (Visual Age For Java.) From VAJ 3.5, you could migrate to VAJ 4.0 or WSAD 4.0. After that, you had to go to WSAD (5.X) (Websphere Application Developer.) Last month, the first version of RAD (RAD 6.0) (Rational Application Developer) was released. While all three names are IBM's IDE to deploy to WebSphere, VAJ was quite different. WSAD asnd RAD are both based on Eclipse. VAJ was based on Visual Age for Smalltalk.
I don't know the release dates off the top of my head, but I imagine you could find them by searching IBM's web site.