I'm 34D, 120 pounds, and 5'2. Am I a good candidate for Breast Reduction?

It sounds so your fit with excessively large breasts. Provided you understand the aesthetic consequences of a breast reduction, namely the resulting scars, you are a good candidate.

D cups vary from manufacturer to manufacturer. In your case, a D cup can cause problems of all large breasts or they may be small relative to body habitus. In any case, see a physician, who can evaluate if your symptoms are truly the result of large breasts. If they are found to be so, then breast reduction surgery will be ideal. Good luck!

Based on your  description of symptoms alone, you may be an excellent  candidate for breast reduction surgery. However, much more information is needed to determine whether you should proceed with surgery, and if so when ( timing).
“Typical” patients who present for breast reduction surgery are women who have disproportionately large breasts, causing problems such as neck/back/shoulder discomfort, postural changes, bra strap grooving, skin irritation/rashes under the breasts, and/or difficulty with activities of daily living and/or exercise etc. There may be both physical as well as psychosocial “stress” caused by the disproportionately large breasts.
Reducing breast tissue mass and elevating the breasts on the chest wall tend to improve or alleviate many of the symptoms associated with the disproportionately large breasts.
Patients considering breast reduction surgery should also consider the potential downsides (risks/complications) associated with the procedure as well. Poor scarring, for example may be associated with the procedure.
When the time is right, I would suggest  seeking consultation with well experienced board certified plastic surgeons. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done.  Communicate your goals carefully as well.
I hope this, and the attached link, helps.

Without being seen in person it is difficult to tell if you need a breast reduction or not. Best to be seen in person.

There is no way we can answer to your question without a deep evaluation, at least some photos. I recommend you to visit a Board Certified Plastic Surgeon and an orthopedist to determine where your problem really is.Good Luck