Sounds like a model 841 (in the 801 series) based on your clues.
1. you say it's a diesel. Diesel was introduced with the 801 series in 1958.
2. You say it has a hi/lo option. That sounds like a Sherman step-up or step-down aux trans. That aux trans can only be mated to a 4 speed transmission, not a five speed. So that eliminates the 851 and 861.
I'm guessing 841 because that's the model I see Shemans installed on most often. I suppose it could even be an 821 or 831, but I kind of doubt it. Does it have a pto and 3 point hitch?