there's all sorts of fun bushings in your involving your rear suspension, trying replacing those with polyurethane units. shitty shocks can also cause this.

this should do the trick.
eBay Motors: ENERGY SUSPENSION MASTER BUSHING KIT 240SX (S14) 95-98 (item 270316068755 end time Mar-10-09 15:51:42 PDT)


wheel hopping is caused by shitty bushings failing to absorb the forces put on the wheel/control arms during hard acceleration.