Kat, I had the 2.8TD in the GQ and swapped it out. It went ok as an empty vehicle, totally gutless offroad, even in low range and I tried every trick in the book to get it going, so i would not recommend that swap. I think the GU with the electronic pump control can be 'chip tweaked' but its a pretty small engine for a big vehicle. I think there are thousands of 3.0L to 4.2 conversions out there, totally possible. I think patrol a Part do a drive in drive out for something like 12-13K?? but a standard swap over engine with no mods would be the way to go I think as there appears to be inherent issues with heat and 'powered up' 4.2's - there are dozens of pages about it here alone.

I would be leaning towards a low mileage 3L motor from the engine series with the least documented variety, can't be specific as don't know enough about the 3.0, however they all can't have been lemons.

Hope this helps or triggers some other comments from those with more specific knowledge.