ok its a nz new nj 2.8 lwb with rear diff lock too

ok so the diff lock is awesome my that has saved me a lot!
next the thing is getting really sluggish i've owned it for about 18 months and it seems to be working a lot harder than it used to.
i'm thinking the auto is on its way out i sometimes have to back off the acc to get it to change gears and it just doesn't feel right, but i'm not going to spend money on the auto i'll be changing it to man. so for my next q i've looked thru the search about whats required and from what i can tell its just gearbox transfer case (as the splines are diff) clutch assembly etc but is that it what about drive shafts etc? surely someone has done this?
and the other thing i was reading about was the egr vavle i've read a few posts about blocking them off, any thoughts on the matter?
its been a while since i last did a economy check but from memory the last time i filled it i only got around 500km to a tank which doesn't sound right and i'm sure that was just normal driving,
Any ideas much appreciated
Thanks in advance Lee