Why won't my 18 speed eaton fuller automatic transmission shift?

Is yours an autoshift, or a fully automatic transmission? With the autoshifts, the synchroniser has a bad tendency to go out because of improper clutch manipulation. This shouldn't be an issue with a fully automatic transmission, however, and if that's what yours is, the first two culprits would be faulty circuitry in the transmission computer, or poor coordination between the computer in the transmission and the engine computer (this was a recurring problem for Allision automatics coupled to Detroit Diesel motors). There should be a way of reading off fault codes from the transmission, but that's going to probably require you to go to a mechanic with the necessary computer software to do such.