You create unique voices by creating unique personalities, unique viewpoints, unique ambitions.
You don't need complex language or different grammar to distinguish a coward from a fearless soldier, a sex-obsessed teen from a reminiscing grandma, a depressed widower from enthusiastic happy newlyweds, a scientific nerd from her beer-drinking carefree brother.
Unique voices can be in what they talk about, their analogies, their attitudes, their ideas (smart and good, or just plain stupid).
Vocabulary doesn't matter that much, make your characters distinct and discernible. You can still give them catch words in simple language; e.g. one says "Cool" a lot.
Aaron stood ahead of the group, on the brink of the canyon, narrowed his eyes, looking across it. He sighed.
"I can't see a way across it, guys. Any ideas?"
Sheila, holding hands with Mark, said, "Bobby can do it. Bobby knows everything."
"I don't know everything, Sheila."
Sheila, looking at Aaron, said "Everything!"
Aaron raised his eyebrows, and looked to Bobby. "Any ideas?"
Bobby joined Aaron at the canyon edge, cautiously looked over it and frowned, then looked across the gap. "See that tree over there? The Oak."
Aaron said, "Sure."
Bobby walked away from the edge, eyes on the ground, and picked up a stone the size of a tennis ball, and hefted it. He approached Mark and Sheila.
"Hey big guy, how far can you throw? Like, a baseball?"
Mark nodded. "Outfield to catcher, for sure."
"Great. I'm going to walk down there, when I wave to you, you throw this stone as far as you can across the canyon. Aim for that oak tree, see it?"
"I can't throw that far, B-Dog."
"I know. Just as far as you can, that'll help me."
Mark nodded, and took the stone and walked to the edge. Bobby trotted 100 yards down the edge of the canyon, and waved to Mark. Mark threw the stone hard. Bobby watched intently as the stone arced high, and fell into the canyon.
He walked back to the group, eyes on the ground, thinking. When he arrived, Aaron spoke.
"What are you trying to figure out?"
Bobby said, "Distance. I know a way. We need to make some things."
Sheila beamed, and punched Mark in the arm; he didn't flinch. "Told you!"
Bobby said, "I have to unravel your sweater though."
Sheila frowned. "Aw."
Mark grinned. "Ooh, snap."
Who's the leader? Aaron. Who's the brains? Bobby. Who's the brawn? Mark, an athlete, he's prone to nicknames and bro-slang. Sheila is the cheerleader. She relates to Bobby like a sibling, and Mark knows him, calls him B dog.
Simple words and sentences are fine; make sure your character personalities and attitudes are distinct, and they stay in character (well, perhaps in extreme stress they step up or chicken out, but outside of such transformational moments).
Keeping language simple is actually a good thing.