>Sound like a question for someone with [font size =4]REAL
>BIG [/font size] trucks...Uh, BigD et al?

Oh sure, put me on the spot. First of all, [font size =4]REAL
BIG [/font size](Class 8) trucks don't have two-speed axles. We just have[font size =4]REAL BIG [/font size] transmissions!

Well,I don't know a whole lot about them, but since you asked. buffy pretty much nailed it on the head. There are two ratios in the carrier. 1:1(a.k.a. DIRECT) and a deep reduction ratio. Most common were the electric motor driven ones. They had that little red push/pull button on the gearshift. An electric motor on the carrier drives a worm gear that actuates the shift fork for the different power flow. You had to make sure the axle did indeed shift before you mashed the throttle, or you could drastically shorten it's life span.
They had to change the ratio in the speedometer cable too. That's how I made sure that the shift was completed, because the speedo would jump when it changed gears.