Try running lower psi in the tyres

I'd bet its an alignment issue. Unfortunately there aren't many adjustments to make on a stock S130. It's not a weight issue though. You just can't make them light enough for that to be a huge issue.

I also doubt it's a weight issue. None of the stuff removed from the car was from the rear based on the opening post. What brand are your tires?

Scientific assessment.

Just for fun (or to clean up the floor) I went into the garage and pushed around my 240Z, my 280Z and my Z31...assuming your 280ZX still falls between the 280Z and the Z31...the 240Z is way, way lighter...

My '76 gives me some hell in the rain every now and then. If I juice the throttle any more than a 'reasonable' or 'sensible' amount it'll spin and/or slide...

... but sometimes it's fun to drift around a corner in the rain... it can also be very scary when you nearly hit a BMW M5... yikes!