EXCLUSIVE

Abigail Ratchford's milkshake dripping, booby bouncing homage to In-N-Out ain't bringing the burger joint's execs to the yard -- they're demanding she take down her viral video ... or else.

In-N-Out lawyers fired off a letter sent to the model's producer, Liverichmedia ... saying they've been flooded with complaints over a video posted last month showing Abby in "various sexually suggestive acts with In-N-Out Burger food."

At first viewing (second, third and fourth too) ... the "Dream Girl" clip might seem like a great plug, but the burger bosses are worried customers will think they're in bed with Ratchford -- which they say doesn't mesh with the company's image.