2007 Buick Lacrosse. The parking brake does not hold when tranny put in drive

Does it have rear drum brakes or rear disk brakes?

I've dealt with this extensively on my own 2007 Lacrosse. The parking brake hardware is built into the caliper, and if you don't use it EVERY TIME you park the car, rust will freeze the hardware and render the parking brake inoperable. Worse yet, the parking brake hardware is not independently repairable -- the entire caliper must be replaced. I'm going to wager that your brake job 10k miles ago didn't include calipers. So probably both rear calipers have to go, and I wouldn't be surprised if your rear parking brake cables are shot too, as I had to replace mine not long ago. A DIYer could do this for a total parts cost of about $200, give or take; expect to pay a shop at least double that to do it for you.