Just been through the same thing. Luckily my local holden blokes not a bad egg and they did it for me just out of warranty.
Pretty much its due to holden using the same box for both the 6 & 8 without strengthening the guts of it.
Story goes (from a transmission specialist) that due to the amount of low end torque put out by the 8's it tears up the 2nd
gear band andmakes it slip on the shaft, and it only grips when it finally catches on, hence, the sudden kick into gear.
All they do is rip out the bands and replace them with reinforced ones (kevlar i think) andgenerally need to change the clutch packs.
I didnt see the bill but it would have been right up there!!
Yeah i've changed the trans and diff oil but my diff contiues to chonk and chatter even after using redline
shockproof oil, in fact making it worst with a moaning sound during a slow turn.
cheers for everyones help.