This is not likely to be a LIM gasket failure. I wouldn't be worried about it in your position.
To clarify, what we have is a rough idle in PARK, but no symptoms in DRIVE.
True: the LIM gaskets is not the cause of this problem....but every 3800 owner should be worried about it if they don't know what conditon their LIM gaskets are in.
1997 Grand prix GT Black Coupe- The beater
1998 Grand prix GT Black coupe- Top swapped, XP cam
2008 Grand prix Midnight blue- The slow but nice car
1999 Chevrolet Silverado red ext. Cab