Kalvin is correct about getting into PT and getting another MRI or other imaging done.
Tylenol 4 IS codiene BUT I will caution you that you should look for other options before trying something stronger. So many people get on a strong narcotic pain medication seeking relief from chronic pain only to end up addicted to it, and I'm talking about both pysical dependance AND psychological dependance. It is possible to have a physical dependance to medication but not be "addicted".
Since you have chronic pain your likely not getting around physically as much as you were before which can lead to the weight gain. The weight gain can cause an increase in your pain because your bones are having to bear that load. Also some antidepressants can cause weight gain as a side effect. I'm unsure about lyrica.
I too take neurontin as well as a narcotic pain med called Norco. I find that neurontin helps with my pain almost as much as the norco. I have not had any weight gain while being on the neurontin.
Talk to your doctor and try to get some help in finding out if your pain is nerve based because if it is Neurontin can help. Neurontin is used for nerve pain. Also you can talk to your doctor to find out if some of the pain is from muscle spasms because there is a medication that might help called flexeril. It's a muscle relaxer but in my experience not a very strong one so it may help.
Have you been diagnosed with depression or fibromyalgia? Lyrica is used for both of these disorders.
Its been almost 2 years since you had surgery? Did you do any kind of follow up with your surgeon? They should have had you back quite a few times to see how you were doing and so they could check your X-rays.
Did your surgeon send you to Physical Therapy? Its almost impossible to get better without it. You need to see another Doctor and get a CT or MRI scan done, they might be able find the reason why your in so much pain. I had L4-5 done about 8 years ago, and it took forever to get better, but 2 years is ridiculous.
Tylenol 4 isn't that strong. Its just codeine right. Tell them to give you something stronger. You must not be from the states. I've never taken Lyrica but I have taken Neurontin and their similar and everybody has a problem with weight gain, its not just you. Tell that Doctor you need help, if he won't help you, find another one.
Take Care