I own a 2000 excursion. When i reach 40 mph an hour and slow down the entire front end start to shake like hell! I could go 60 and will probably not as bad as if i hit the brake to slow down. The original tires are p165 but the previous owner put p175 . Could it be just an alignment? Too much play in wheel even before braking.

You have worn front end parts like tie rods - you need a complete front end inspection