other than the housing pretty much transmissions work the same way
the lift trannies are actually simpler since they only have a fwd and rev gear (some bigger ones have 2nd and 3rd your 40 may have a 2 speed)

those 40 nissans are built by MCFA right ed?

one my customers just gave me a letter he got from mitsu about an issue with the smaller nissan built mitsus with a similar problem
though this may not relate to yours

"the mitsu by reference... blah blah, may experience a fatigue failure of the Belleville spring, an internal transmission component designed to reduce trans engagement shock. If this was to occur, it may cause the vehicle to continue to travel in the mechanically engaged gear even though the lever is in neutral"

obviously my customers is handled through a safety recall for free by mitsu regardless of warranty status ( this particular unit is an 04 and is well out of its warranty even on emissions) to prevent gigantic lawsuits from guys driving off docks due to mechanical failure

post the serial number and full model number of the machine
(or at least the prefix of the s#)