4.6 diff versus 4.3

[dom14]

You just don't get the engine braking (being an rb30, not much anyway). My car for instance, I don't get that backpressure/engine braking until 2.5/3k rpm. Those rpm on 35's compared to 31's is significantly faster. 4.6's may bring that back into line, but to be honest, might still be too quick anyway.. It hasn't been a big problem, more an annoyance...

I'm not 100% sure, maybe someone else can clarify, but didn't 2.8GU come with 4.8 diff gears? If so, couldn't you just transfer the centre?
Yes both front and rear diff needs to be the same ratio.

How much lift do you have?

2.8GU came with 4.6 diff(and some may be 4.3)
4.8 crown wheel and pinion is an after market Italian import, according to the internet.
Yes, AFAIK, 2.8GU diff is H233B, which is a straight swap for GQs.

I'm not sure the exact amount of lift on mine.
I have to take a guess and can say it's above 2".

You get the engine braking by switching from higher to a lower gear than suitable for particular speed and revs go high as a result, while car slows down. I'm not sure what you meant by engine braking not happening until 2500/3000rpm.