There are differences in the front hubs and driveshafts used, but overall they are all 1984 Chevy's. The Mil contract required them to be the same for parts interchangeability purposes and the model year upgrades were not applied to the later trucks. There were some late contract 87 trucks and some early contract late 83 trucks, but that's it.
There are some CUCV IIs that were procured in the early 90s, but there wasn't a big 50k plus contract for them like the original CUCV series. There are also some LSSVs floating around that are late model Silverados, but again, they were a limited purchase.
Later,
Joe