by David Dobbs Don't look now, but you might have recently switched one of your most basic lumber products — 2x4s and 2x6s — from western to eastern lumber. Particularly across the Southeast and Midwest, changes in quality and price have driven lumberyards to replace western and southern sticks with eastern sprucepine- fir 2x4s and 2x6s. The strange twist to this story is that some dealers aren't telling customers they've switched framing lumber, despite generally good results. "It's not a concern about quality," says one Michigan lumberyard owner, who quietly made the switch to eastern s-p-f in 1992. "We've checked with the builders, and they've had good results. But some builders have prejudices about eastern lumber. So I don't tell unless they ask." The main reason dealers are switching