The previous owner of my house chained the sewer drain line for my washing machine with one of the master bathroom sinks. Because of this, when the washing machine drains, it causes the sink to gurgle, and occasionally forces sewer-gas out. This is a rough diagram of how things are setup: 
What is the best way to fix this? Do I need to take both lines all the way back to the vent pipe, or is there a way to fix this while still chaining these drain lines?
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More cowb... venting!
The washing machine needs its own vent circuit, extending vertically from the trap, which would connect to the vent stack independently. (You haven't drawn a trap, but I assume it has one, which is apparently an illegal S-trap.)
Then, you also have an illegal S-trap on the sink. It should also have a vent circuit (which could merge with the laundry vent circuit). That alone may solve your gurgling issue, but you could still be getting gas at the laundry drain without a trap seal.
As Ecnerwal pointed out in a comment, a laundry drain trap is usually much lower than I drew it here. it's often just above the floor. Also be sure that the vent is adequately above the flood rim of all fixtures on that floor level.
isherwood
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