There are alot of model that have the rock catcher in front of the rear
wheels. Fords Maquis & Crown Victoria in 2001 eliminated the lip around
the rear wheelwell openings.
Check with a Chevy parts department, I think in 2001 the rear wheel
wells had a plastic liner that fit right up gainst the the area in
question. You may be able to add it to your 2000
1991 Pontiac Bonneville LE