Starting with the black or gold dubsta 2, you'll want to swing by los santos customs around burton or la mesa during daylight hours. Driving another dubsta definitely helps with spawning and if you've got casino access, ms baker can hook you up with a black one to increase your chances. The rare versions tend to show up more when you circle the shops a few times, and switching sessions can refresh things if you're not having luck.

For the sandking xl, your best bet is cruising near larry's rv sales out in sandy shores, especially between 9 and 5 in game time. Taking another sandking or any off road vehicle makes them pop up more often, and you might need to loop around the area a couple times before finding one.

Good to know: something interesting about both vehicles is they can be pretty stubborn to spawn so don't get discouraged if it takes a while. When you do spot one though, make sure to grab it quick and get it stored since they tend to disappear if you wander too far. Might be worth trying different sessions or completing a mission if you're having trouble, sometimes that helps reset things and give you better odds.