Since Whitley Wyatt retired in 2000 after 33 years as a trucker, he’s collected a pension of $3,300 a month. Now, the 71-year-old says as much as $2,000 of his monthly check is at risk because of legislation passed by Congress last year that is meant to help underfunded multiemployer pension plans bolster their finances by giving them a way to cut benefits for some retirees.