The 96 Montero will have a 9.5" ring gear if SR and 3.5L or 9" ring gear if not SR and 3.0L. These differentials will not drop into the pickup rearend that uses an 8" ring gear. You can readily find a 1997-99 Montero Sport with either of the engine sizes and match to the 96 Montero. The Sport rear axle will have disc brakes, too, and the OE leaf springs will give about 1 inch lift over the Max springs. This may require 16" wheels, too, as some of the OE 15" rims do not clear the rear caliper of the Sport axle. Regardless, the Sports often had 4.900 gears that trump the bigger 96 Montero. There are many discussions of the Sport variants that include hybrid LSD/air locker, Torsen locker, and normal old LSD. I hope this wordy answer addressed the meat of your question.

The 2.4L engine is an upgrade from the 2.6L, especially just going carb to FI.

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Russell
aka rocks in head