Perhaps you are conflating two different stories? Parts of your description match "X Marks the Pedwalk", a 1963 short story by Fritz Leiber, first published in Worlds of Tomorrow, April 1963, available at the Internet Archive.
There is no harvesting of organs or stamping of foreheads, but there is that X in the title, and here is the opening scene:
The raggedy little old lady with the big shopping bag was in the exact center of the crosswalk when she became aware of the big black car bearing down on her.
Behind the thick bulletproof glass its seven occupants had a misty look, like men in a diving bell.
She saw there was no longer time to beat the car to either curb. Veering remorselessly, it would catch her in the gutter.
Useless to attempt a feint and double-back, such as any venturesome child executed a dozen times a day. Her reflexes were too slow.
Polite vacuous laughter came from the car's loudspeaker over the engine's mounting roar.
From her fellow pedestrians lining the curbs came a sigh of horror.
The little old lady dipped into her shopping bag and came up with a big blue-black automatic. She held it in both fists, riding the recoils like a rodeo cowboy on a bucking bronco.
Aiming at the base of the windshield, just as a big-game hunter aims at the vulnerable spine of a charging water buffalo over the horny armor of its lowered head, the little old lady squeezed off three shots before the car chewed her down.
From the right-hand curb a young woman in a wheelchair shrieked an obscenity at the car's occupants.
Smythe-de Winter, the driver, wasn't happy. The little old lady's last shot had taken two members of his car pool. Bursting through the laminated glass, the steel-jacketed slug had traversed the neck of Phipps-McHeath and buried itself in the skull of Horvendile-Harker.
Braking viciously, Smythe-de Winter rammed his car over the right-hand curb. Pedestrians scattered into entries and narrow arcades, among them a youth bounding high on crutches.
But Smythe-de Winter got the girl in the wheelchair.
Then he drove rapidly out of the Slum Ring into the Suburbs, a shred of rattan swinging from the flange of his right fore mudguard for a trophy. Despite the two-for-two casualty list, he felt angry and depressed. The secure, predictable world around him seemed to be crumbling.