The serial number on my old jeep truck is 3406X10258. What year is it?

It's 1965 : the letters W,X,Y,Z in the VIN were introduced for 1965, and after 65' the serial numbers begin either by "2" or by "1" but with 6 digits, not 5 like "10258", so it's a 65 VIN. Decoding : J3000 4WD, 6000 lbs GVW, Tornado engine.

I couldn't find the exact year from your serial number only a year range which is 65-71.

Your serial # breaks down as follows.

34 = 1965 - '67: J-3000 4WD, Gladiator or 1968 - '71: J-4000 4WD, Gladiator
06 = 1965 - '70: Cab & Chassis, Truck or 1971: J-2500 Cab & Chassis, Truck (This means it did not come with a bed from the factory.)
X = 1965 - '67: 6000# GVW or 1968 - '70: 6000# GVW, also console shift Wagoneer (1414X) with exterior metal type feature panels or 1971: 6000# GVW, J-4600, also console shift Wagoneer (1414X) with exterior metal type feature
1 = 1965 - ' 71: Left Hand Drive

You should be able to narrow it down a bit more, however it's going to take comparing pictures of the various years within 65-71 and looking for changes to the style such as grills, headlights etc.