As best as I have been able to determine, the 4JG2 shares a block with the 4JB1 (which strongly suggests that the transmissions are the same) and achieves the increased displacement through bore and/or stroke changes.

The 4JG2 is supposedly indirect injection, as opposed to the 4JB1 with direct injection, and as a result the heads (and internals) are different. There may also be 4JG2 engines with electronic injection control.

Again, this is all an (educated) guess based on the research I've done.

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com