The two speeds are usually "slow" and "really slow!" Those rear ends were used to get maximum gross weight loads moving, and pulling steep hills where the number of gears available could be effectively doubled to minimize the wide gaps between transmission gear ratios. 216's and early 235's didn't have a great deal of torque, so they needed lots of gears. Another option on the big rigs was a "Btownie"- - - -a 3 speed auxiliary transmission between the main box and the rear end. Most of those boxes offered an overdrive ratio to increase road speed when running unloaded.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!