We are tentatively planning a beach vacation to South Carolina.
We are looking for a quiet beach - don't need shopping or golfing or amusement parks or fine dining. We just want to rent a vacation house and hang out. Looking for something along the lines of St. George Island in Florida if you are familiar with that.
Any suggestions?

If you go to the NORTH Myrtle Beach area, this may fit your bill. It's a ways from the "jungle" of the amusements of Myrtle Beach in a quieter residential area. We absolutly can't stand the downtown area of Myrtle Beach, but we love this area.
I have a hotel recommendation for you as well. www.forestdunes.com
There are also other hotels in this nice area.

Thank you!

Hands down - either Kiawah or Wild Dunes in South Carolina over Myrtle beach! I've been to both several times and love them. Both are beautiful, relaxing, low-key beach resorts full of all types (and prices) of vacation rentals...condos, villas, houses. You'll stay on a budget if you aren't right on the beach but that's no problem because where ever you stay is always a short walk or bike ride (lots of trails) to the beaches (and the sand is perfect for riding your bikes right on beach). Charleston is just an easy 1/2 hour trip away from either resort town. I can give recommendations for vacation rentals we've used in the past if you like.

Edisto Island.

You might also look at Fripp Island, near Beaufort, S.C. Easy to walk in 30 minutes. Many, if not most, get around using golf carts. Numerous marsh deer. Closed to all but owners and vacation renters/guests.

I live in Columbia, SC...I would suggest Litchfield or N. Litchfield for quieter beaches...Surfside, Myrtle, or Garden are more touristy..What is your budget?...How many people? Dates? I have some friends that have rentals that are nice and you can talk to them directly and save fees...

Touristy is the last thing I'd call Garden City.

Try Pawleys Island, Edisto Island or Fripp Island.

We love Fripp Island. It is a private gated island just 30 minutes from Beaufort. By map very close to hilton head, by car about 45 minutes. Can rent a place with an "amenity card" need to show the amenity card when dining to keep the public off the island. Great beach, very small island, drop the car, golf cart preferred method of travel. great pools totally relaxing- two golf courses.

I agree with just about everything that has been said above--you probably don't want downtown MB, but surrounding areas are less crowded and less touristy.

Also, I know you said SC, but if you could go just a bit further north, I am thinking Sunset Beach, NC might fit the bill. It's a small barrier island with almost all houses, no high rises, just awesome unspoiled beach.