Hi Mbricken,
Welcome to the Mattress Forum! 
I’m sorry you’re not sleeping well with your Tempurpedic mattress. There is some information about the many different symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful, but the most common reason for lower back pain is a mattress that has comfort layers that are too thick/soft or a support core that is too soft, which on the surface seems to be what you’re describing with your past results and your brief switching of mattresses (a simpler innerspring mattress with less comfort material).
There is a good reference article for the different sleeping positions and how that can impact what type of mattress might work best for you here.
While I certainly can help with the “how” to choose, there are too many unknowns, variables, and personal preferences involved that are unique to each person to pick the “what” or to use a formula to predict a style or specification for any individual that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here), because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress.
I would recommend you start by reading the mattress shopping tutorial here. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you, and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you, including the price as well as the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for.
Outside of PPP (Posture and alignment, Pressure relief, and your own Personal preferences, which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
If the issue is that your mattress is too plush or unsupportive for your needs, adding a topper would not cure the issue for you and I would not recommend it. Even adding a basic polyfoam pad tour existing mattress didn’t provide positive results.
Subject to first confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent (see this article) and to making sure that any mattress you are considering meets the quality/value guidelines here … the better options or possibilities I’m aware of in and around Birmingham, AL area are in post #57 here.
Phoenix