Ram sold 120,026 pickups in the first quarter, while Chevrolet dealers moved 114,313 Silverados. Wind the clock back one year, and Chevy's lead looked almost unassailable: 135,545 Silverados were sold in the first three months of 2018 compared with Ram's 103,964. Ford's F series, gunning for its 43rd consecutive yearly sales crown, barely changed, going from 214,191 in the same period in 2018 to 214,611 this year.

In addition to being the industry's top-selling nameplates, full-size pickups are crucial to U.S. automakers' profits. Ram displacing Silverado as the industry's second-best selling model is all the more remarkable when you consider that truck buyers rarely change brands.

"You've got a fairly high degree of loyalty in the full-sized pickup market, so you don't have a ton of customers who are in play in a given month," said Mark LaNeve, Ford's vice president of marketing, sales and service. "There's really a small part of the customer base that is susceptible to brand switching. With our leadership, that provides somewhat of a competitive moat around our business," he added.

IHS Markit, which studies loyalty data, doesn't yet have enough information to explain whether brand loyalties have changed. "It's just too soon to be sure if it's a loyalty shift or a production capacity issue, or a bit of both," said IHS analyst Stephanie Brinley.

At the corporate level, General Motors still owns second place thanks to its GMC Sierra, the Silverado's upscale cousin. GMC sold 40,546 Sierras in the first quarter, down slightly from the 41,465 sold a year ago.