Gear ratio and mounting should be the same, but the brakes are different. They changed the rear brake shoes around some in 81. That should not prevent you from using it though, just have to remember to buy brake shoes/parts for an 81 if you ever do the brakes on it. The only thing I'm not totally sure of is if the e-brake cables would be the same, but I am thinking that they'd fit.
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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
