Does the exhaust pipe have to be removed in order drop transmission pan while replacing filter on 2000 Chevy Express van with a 5.7 liter engine?

No, you generally do not need to remove the exhaust pipe to drop the transmission pan on a 2000 Chevy Express van with a 5.7L engine, even though it's a tight fit in that area. There's usually enough clearance to maneuver the pan down and out. You might need to loosen some exhaust hangers to give yourself a little more wiggle room, but full removal shouldn't be necessary.