I Have 2001 Ford Focus An only My Rear In Tires Slide when its Raining or Snowing?

Could it be the tires or something else. Its don't slide as much on street. But on the highway it slide enough were i have to slow down a Lot

It's the tires. If your tires are poor quality, or they are worn to 4/32 of an inch or less, your Ford is simply hydroplaning. Get a new pair of quality tires to mount on the rear of your Focus as soon as possible.

Its called Aquaplaning

Your tyres arn't keeping in contact with the road due to a layer of water between the tarmac and rubber. Check your tyres aren't bald as the grooves are there to stop aquaplanning by lets the water pass via them.

The faster you go the worse it gets.

It could be tires but my guess is that you have an alignment problem. Get the alignment checked first.

I would guess it is your tires, A good tire store will give you an inspection and help you determine the issues and the solutions.