I removed the elbow and drained and ran both sides and they drained fine. But when I reassemble and run the disposal it fills up again. I can use the sink, but not with disposal running
From your original post it really sounds like the DISPOSAL is clogged and not the line.
What happens if you put a bucket under the disposal, disconnect the elbow from it and run water through it from the top, does it come out into the bucket.
The cap on the ground may be the cleanout that goes to the septic tank. Cleanouts for the house plumbing are usually up higher I thought, not at ground level.