As much as I love Steve Winwood and the Dead he was never a member of the band. Traffic opened for them and they did Dear Mr. Fantasy together but that's about it.
NashvilleBlues might have been confusing Steve Winwood with Bruce Hornsby?
While Steve Winwood was never a member of the Grateful Dead, they did share the stage more than once - whether it was just Jerry sitting in with Traffic, or Winwood sitting in with the Dead.
From Jambase: Grateful/Winwood
"In 1994, Winwood and Capaldi (Wood had died in 1983) reunited to reform Traffic. The short reunion lasted the summer while the band opened for the Grateful Dead.
On multiple occasions, Grateful Dead guitarist Jerry Garcia sat-in with Traffic during their opening set on the Dead cover staple “Dear Mr. Fantasy.”
Garcia’s sit-ins in the summer of ‘94 were not the only times Winwood shared the stage with the late, legendary guitarist. Back in November 1970,
Winwood sat-in with the Grateful Dead at The Fillmore in New York City. Traffic was in town to play The Fillmore on November 18 and 19, 1970"._____________________________________________________________Rip this joint, gonna save your soul, round and round and round we go......