Box blade is more of a problem unless your trying to cut and fill your low lying area. Disk or spring harrow if your tractor is up to it.
Check with your County Extension Agent through your Agricultural College if you have one. They will have a better idea of what preparation works for your type of soil and proper grass seed. A good fertilizer schedule, proper drainage and correct carrying capacity for livestock will keep a pasture healthy. The fertilizer will promote root growth and help with the compaction problem.
I live in the "Grass Seed Capital of the World" and nobody I know plugs their fields. Golf courses, playing fields, and lawns but not pasture. The underlying clay layer "one to two feet" necessitates tiling in some areas.