My turbo system was fitted by CRD in Alberton by 2 owners back. I'm not sure what turbo was put on then, but the previous owner changed it to a bigger intercooler & turbo ( make unknown ..... probably Chinese....
) as he used to pull a big sea boat with the Patrol. Sand and a diesel are not the best friends to start with and our "old school" technology motors are even less happy in sand..... I'm talking about beach sand, however I did Atlantis Dunes in the Cape with my Patrol, wit virtually zero dune driving experience, with 33" Kumho Muddies on and yes I battled, but I did everything the others did. The turbo on mine on pull off shows itself at about 1400 rpm and pulls nicely till 2800 rpm. After that the flywheel on these motors is so big that the turbo cannot help much more I think. On the sand, in Ponta Do Ouro ( Mozambique) I very seldom push harder that 2200 rpm unless storming some of the steep climbs/tracks that don't get used regularly.
You might be under fueling when the turbo kicks in. Hows your EGT's when driving up a long steepish hill on the highway?