Relegated Stafford gave Cambridge a shock as they came from behind to win and dent United's play-off bid.
Ben Farrell's stunning 30-yard effort gave Cambridge an early lead but the visitors went straight down the other end and levelled through Neil Grayson.
The Us had the majority of the ball early in the second half but did nothing with it, and paid the price.
Nathan Smith's 64th minute goal ensured Rangers' first win since February and ensured the hosts remained third.
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