not sure about the muscle soreness.. but the lower back pain can be due to kidney issue (you might be detoxing too fast the kidneys cant handle it).. you may need to slow down detox or takey kidney herbs from MH
I have had personal experience of this and also adviced a friend who have had similar issue... lower back pain went away when she stopped juice fasting and took kidney herbs..
for your case since you are eating the fruit whole.. you probably just have to take kidney herbs rather than replacing the fruit diet...
anyhooo... MH can probably be able to give you a more sensible advice (the above is just based on my experience)