Where the heck are you supposed to lift the rear of the car from? Looked in the manual, only thing I can find is the chassis rails. I'm tempted to use the rear diff, but kinda skeptical. I need both wheels up (installing an exhaust). What points have other people used?

there is a support brace the attaches to the underside with a knuckle. Its a very strong point, and can be found just before the rear wheel opening.

I too am reluctant to use the bottom of the rocker panels.

Get a pair of wheel ramps. Did wonders when I installed the Borla.

Picked up a set of low-rise ramps.

The differential is the suggested jack point for the Miata. I see no reason why it couldn't also be used for the RX-8. In fact, when I changed the trans and diff fluid on my 8, I did use the diff for the floor jack to lift it up (held it up with jack stands under the side sill jack points).

Regards,
Gordon

Ramp were $20 at Kragen. Well worth it, imho. Glad they finally had low-rise ramps, my old ramps were only good for my tundra. Supra, miata, & rx8 won't clear 'em.