FMTV stands for Family of Medium Tactical Vehicles and refers to all of them, regardless of which model or weight capability (M1078, M1083, etc.).
LMTV stands for Light Medium Tactical Vehicle and refers to the 2.5-ton/2-axle version. (M1078, 1079, M1080, etc)
The 5-ton/3-axle versions (M1083...