I have a 1999 3.5 Manual sedan, and when I'm trying to push my right foot through the floor, as I roll along I have noticed recently there is like a weird noise halfway between a squeak and a clunk coming from the arse end.

I'm wondering if I am on the right path thinking that I need to replace some bushes around the suspension?

Can't hear it inside, only when I have the window down and its not very loud at all, I heard it in an odd moment when I didn't have some frightfully uncouth music blasting...

thats the first thing id be checking (bushes) or it could be a hand brake sqeek/clunk....... but as far as you pushing your right foot through the floor that another problem all together hahaha.
i know its common for centrivical (spelling) bearings to in the rear end they usually cause a wobble in the rear while cornering or in the wet while towing (personal exp)

frightfully uncouth music blasting...

Haha love it.

Check your bushes for cracks/fluid. Check your handbrake shoe for wear. I bet its one of them. WD-40 is your best friend here.

I'd also check the exhaust especially the rubber hangers at the rear....should be easy to check simply by trying to move the mufflers by hand and when cold!

Cheers for the advice guys, I have to futz about removing mud flapps this weekend so I will nose about with some wd40 while I'm down there :D

last time I had a similar noise it was the bushes on the sway bar clinging onto the girder between the rear wheels on my VR commodore, very similar noise.

Exhaust I will check also, thanks!