Dwayne Johnson -- Massive Rock Warning

9/7/2010 10:30 AM PDT
Dwayne "The Rock" Johnson found some time to hit an L.A. gym in between shooting scenes for  "The Fast and the Furious 5" -- and from the looks of this photo .... he's playing the role of a diesel truck.

As for the Miami shirt -- dude used to play football for The U back in the day ... and was a member on the 1991 National Championship squad.