G'day Paul,
I've owned two 3 litre Patrols, a GU II (2000) and a GU IV (2005). Both travelled over 100,000 km with me before they moved to new owners. Mostly trouble free.
Earlyish model GU's had a weak 5th gear spline, which would let go but I believe this was fixed in the early 2000's. Stories of grenades (engines blowing) still exist but I don't know anyone who suffered, certainly I didn't.
A mate with a GU III had his dyno'd and was told that the MAF sensor was faulty and the engine had been over boosting. The mechanic advised that it was not possible to determine for how long, or what if any damage may be lurking, so my friend now drives a Prado.
I now drive a Patrol Recovery Vehicle, a 200 Series Cruiser but that's because I was sick of dealing with Nissan, not because the Patrol is no good.