I agree with yooperom, it's time for the meds. We did metacam with our GSD this past year. G/C would have been more effective years ago; not so much now. People think it's the "hips", but actually it's usually more a problem of the spine, which interferes with functioning of the whole rear end. It's a degenerative process, and, unfortunately, once it gets to the point you've described, it progresses fairly rapidly.

We've gone through it now with two of our dogs; one, we just lost in July. (I had a thread about it if you want to look at it.)
I would definitely recommend the harness over the towel. I was so thrilled when I found a harness had become available between the time of our dogs having this issue - it's a back saver. At some point, though, the rear end can become completely useless and getting them around becomes very difficult, even with a harness. (Many have made the decision to let them go by this point.) We chose to get wheels for both of our dogs, since they were still enjoying life and even chasing the ball in their carts. Generally pain isn't an issue; they're numb in the rear. Yes, there's some general arthritic pain, but that's likely been there for years and doesn't necessarily increase a whole lot. Again, it's a degenerative issue (usually) so it is progressive. Many, many large dogs get this.
ETA you can sort of see in this pic that we kept the harness on her (near the rear) so we could help move her around. It was so helpful.