Like oldisuzuist said, 4x4 LUVs didn't show up until 79, then in 81 they changed to a completely different look. If you look through the gallery on the main site here you should be able to find a 1981 and confirm it wasn't 81 or 82. The big 4X4 decals where available either year and I don't think there was much difference between them if any.
The only visible distinguishing trait I can think of for the 79 vs 80 is the side marker lights. They are rectangular and there is one amber on each front fender and one red on each side behind the rear wheels. The difference is the the 79 is just like the 78 and older. They have a shiny metal ring that goes around the outside of the lens. Not chrome, but somewhat shiny. The 1980s did not have the ring, and the lens had a slightly rounded over edge. The 80's don't seem to stick up quite as much. I don't know if I can find a picture of both types, maybe someone with a 1980 can post of a pic of their marker lights. The 79 and older are of course more common since more years had them. You'd have to be pretty observant to notice and remember the marker lights though. Very few could spot the difference from more than 10 feet.
There is another few differences but most are not visible. The voltage regulator connector is round on the 1980 and rectangular on the 79 and older. The clutch plate is bigger on the 80. The only other thing I can think of is that the 80s have a 4x4 indicator light in the gauge cluster (in the speedo I believe, but not sure). If you remember seeing a green 4x4 light on the dash come on when it was in 4LO or 4HI, then it was an 1980.
If you can't pin down any of those details, then the best you can do is look for a the right body style. 79-80 or 81-82 depending on what matches. They are so close that other than matching the year for sentimental reasons it is almost impossible to tell them apart. Good luck on your hunt!
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
