96 mack mr688, allision ht740, no reverse and truck dies when put into forward gear.

got a weird issue here.
my 96 mack mr suddenly lost reverse.
this is what is happening.
no reverse, no backup camera.lights,or back up alarm. when i try forward gear truck dies.
this does not happen all the time. some times it goes into drive and engine stays running.
with the truck off, if i toggle between reverse and neutral a couple times and then start the truck, it goes into reverse,but only once it wont go into reverse again????
change the tcm, some relays, backup switch on pass side of trans. filters etc...
thinking maybe the input speed sensor is not jiving with the output speed sensor.
anybody have a similar problem? hoping its not a trans. oil looks new.
any help would be great.

 

what do you think maybe nss or ign switch?