There is no such thing as a "manual" shift 2 speed axle- - - -at least not one that's been made since the 1930s. They're either vacuum or electric shift. You would need to swap the complete axle assembly to an electric shift type if you can locate one, along with the electrical components to make the conversion. Then there would be the problem of finding an axle with compatible high and low gear ratios for the Diesel engine's torque band and the number of speeds of the transmission you'll be using. It will be completely different than the gas engine you're replacing.
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!