Jay Z and Beyonce -- We Got Hillbilly Swag ... In Hawaii!! (TMZ TV)

They've got enough cheddar to BUY their own island, but Beyonce and Jay Z went low key in Hawaii ... cruising around in the bed of a pickup truck. If they were trying to fly under the radar, it didn't work.

Sorry, Jay & Bey -- can't hide that kinda money ... even in a truck. ALOHHH-HA!