I have a 2004 Buick Park Avenue that has about 145,000 miles. Problem: It shifts hard after it warms up. Why and how can I get it fixed?

Have someone hold the brake and shift it from park to reverse to drive while you look into the engine compartment watching the motor mount and transmission mounts, you shouldn't observe a lot of play, if you do they may need replaced. If not it could be the clutch plates internal to the transmission. Mounts are not cheap but compared to a transmission rebuild, they are. hope this gives you some direction, best wishes.

You may want the person to hold break and put in drive and lightly apply gas to really see if engine raises excessively. What is excessive? More than a fraction of an inch (3/8 or more) the mounts are metal and bonded rubber and besides holding the engine and trans in place they absorb shifting if you have one that tore, when the engine shifts it can be felt .