We have a 1-story slab-on-grade house built in 1993 with two separate bathrooms. Occasionally we will hear the toilets in both bathrooms start to bubble, and I am sure it is bubbing because it has been significant enough to get the underside of the toilet lid wet. Today when we had the washing machine running we heard the bubbles again so we shut to washing machine down. The toilets bubbled, but in addition, the tub closet to the end of the line start to fill with backflow, not pleasant. I am guessing that this is likely due to a partial blockage outside of the house, we are connected to the city sewer system. The ironic part is that the house next to us had the same problem two weeks ago. The plumber snaked the line out to the street and told them that they likely had a root growing through their line and that it would cost 6000 bucks to dig up the lawn and replace it. I don't even think it is 50 feet from the house to the street. It seems likely that both of our houses could be experiencing pipe clogs/cracks at the same time since they will built at the same time. But I was wondering if it was possible that something in the city sewer system could be causing this. Any input is appreciated!

Most likely you have roots also. As a rule,(unless there's a break in the line) you'll locate it at the city raiser where your sewer connects to the city main.
First get the line snaked and cleared and address the root problem until you can dig up the raiser and get it repaired.
RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months. Good luck, Tom

I am a little confused, is the riser the line running from the house to the city main? The other issue is that I can't seem to find the trap that would be on the outside of the house. We live in Florida, and I think I read that you mentioned the trap should be within 18 inches of the foundation. I liked over by the water supply lines thinking it would be there and it wasn't. The closet thing I could find was a 4 inch pipe located about 5 feet up the side of the house. When I removed the cover it didn't look like an access to the drain. Also, will the RootX or Copper Sulfate kill the entire tree or just the root, because the tree isn't on our property. Thanks for all of your help.

The riser is the spot the line from the house connects to the main. If the main is tapped at the top of the pipe a length of pipe is added before it turns toward your house to get it to the proper height.
I read speedballs answers and I think he said about a CLEANOUT within 18 inches from the foundation.
The cleanout doesn't have a trap. It has a cap that you can take off to look directly into the sewer line.
You can tell the difference between a cleanout and a drain by looking to see water. A floor drain has a trap so you see water. A cleanout should not have water sitting in it.
You shouldn't have a trap outside.
Lots of plumbers have cameras that will televise the line to get the exact location of the roots.
Each municipality has different rules for what they take care of in the sewer line.
My area the City takes care of the line from the main on the street to the connection at the property line.
I tried to start a Root-x program and we televised ten lines then used Root-x and televised every two months.
We didn't have much luck so we gave up on the program.
I know some areas (Calgary Canada) that use it regulary and I haven't heard any negative comments from them.
To answer your question. No, Root-x won't kill the whole tree.
Speedball is located in Florida so he knows more about the local regulations than I do.
But in my experience the best thing to do is get the line televised. If the problem is just roots, liners can be installed right from the house to the main without digging anything.
I have had dozens of liners installed and as long as the pipe is structurally sound it is a lot cheaper and less damaging to the landscaping in your yard.
One other trick to find out if the 4inch pipe is connected to the sewer line is to take the cap off and have someone hit a manhole cover with a hammer. You will hear the noise quite clearly.

First thing, don't call your neighbors plumber. Get it snaked by a company that has a SeeSnake (television) and they will tell you if there is a broken pipe or just roots at the riser. They can use a cutter on the roots or dig at the exact spot of a break, no where near $6k. Follow up with a dose of copper sulphate down the drain twice a year, it kills roots. You can buy it at the garden supply center.