If I got to the doctor and tell him I'm worried my testosterone levels are low, will they give me deca-durabolin?

Not likely. Deca-durabolin is brand of nandrolone, a synthetic anabolic steroid. Most doctors won't prescribe you any steroid w/o testing first. Even then, most will only prescribe testosterone which is what your body produces & needs. No need for nandrolone.
Joint pains.... If you are on testosterone replacement therapy, you physican may be more inclined to prescribe deca if he is aware of your joint mains. Deca acts as a joint lubricant, and can help temporally with these symptoms.