[unclebilly]

A couple things...

As was mentioned, the blockage is likely between your house and the curb. The reason it comes up in your shower is due to technology developed in the last 2-3 years (Gravity)...

I have never been to your place but I'm pretty sure your shower drain is 2-3 feet lower than your sink drain. Because you have a blockage between your house and curb, the water from the sink flows downhill (this is how gravity works) and since it can't flow out to the sewer line as fast, flows back up your shower drain.

You are lucky you caught this and don't have a bath upstairs.

Likely the problem is roots getting into your sewer line.

A cheap alternative to getting the camera services out is to buy a 30' USB camera off of Amazon or eBay.

Something like this: DBPOWER 2Million Pixels 5M USB Waterproof HD 6LED Borescope Endoscope Inspection Tube Camera these are $22 for example.

We used one to sort out this exact problem at my dad's place 3 years ago. The roots from his grape vines had grown under the house and got into a 4" PVC coupling and were causing the blockage. Rotorooter had angered it out for him. We were able to use the USB camera to precisely find the trouble spot, cut the concrete slab in his garage, dig down the 2-1/2 feet, and repair the coupling with a Fernco and then fill in the hole with concrete.