I believe most of the 1.9L Troopers had a 4spd in them. What you are looking for is a 2.3L Trooper, 86 or 87. Basically just find a Trooper with a carb (not fuel injection) and a 5spd and you should be set. The 5spd trans is longer than the LUV 4spd though, and mounts a little differently. You'll need the driveline shortened and a little bit of welding/cutting for mounts, but other than that it is pretty much bolt on.
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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