Thanks for the query.
It can not be shingles
as shingles doesn't involve whole back and never involves both sides of back.
It is not Scabies as it doesn't manifest this way.
Liver diseases,thyroid dysfunction
,anemia etc may cause itchy back so you may rule them out by getting investigated for them.
Pets in home like cats and dogs may also cause itchy back.Cats may cause cat flea and dogs may cause dog dandruff
The most likely cause could be due to dry skin
which is very common.
Take the following measures if the skin is dry.
Moisturize your self well and do it right after the bath so that moisture of bath can be trapped.
Don't bath with with hot water.use only warm water.
Humidity helps in dry skin so use humidifier in your house.Central heating leaves your skin dry.
Avoid using harsh soaps.
Wear cotton fabrics.
Use anti itching medicines ( anti histaminics ) in the night.
If these measures don't help,you are advised to consult a dermatologist
for the solution.
I hope to have answered your query however you may revert to me for any further query.
Best of luck