1973 -Different from earlier years, the frame rail thickness was increased. The fuel tank was moved from inside the cab, to the right side frame rail. Mounted on the outside of it, which caused much controversy over the years. (This began the "exploding gas tank" fiasco in the 1990's). Vehicle wheelbases were increased by 2 1/2". Increasing the wheelbases allowed the new crew cab trucks to make their debut. Crew cabs were an option for all 20 & 30 models. The "Big Doolie" made it's debut this year. Rear suspension used leaf springs instead of coil springs. Rear shocks were now staggered to reduce wheel hop. C/K 20 & C/K 30 models had an optional Eaton locking rear differential. It would lock the rear axle when there was a difference of 100 RPM's between the two rear wheels. It was governed, so that it would not lock at speeds above 15 MPH. The front spring rates were reduced to give a softer ride, and a sway bar was added to improve handling. Fulltime 4wd made it's debut on all V-8 auto trans equipped trucks. All inline 6, and manual trans equipped models used the NP 205 "bulletproof" gear driven transfer case. The grille had an emblem with the engine size on it. Blazers were available in 2wd and 4wd models. The one ton model (30 series) was NOT available yet in 4wd. Standard model pick up bench seats did not flip forward.