Most tomatoes (but not all) are "indeterminent" meaning that they will continue to produce new vegetative growth and fruit as long as conditions are favorable, like a lot of other annuals. So lower leaves that wither, turn brown, and fall off is to be expected I think. They are the oldest, and the plant is in some sense "self-pruning". I'm thinking about geraniums and they loose some of their oldest leaves and stems as well, but still continue to produce
flowers. So I don't think it's anything to worry about so long as the new growth looks healthy.
I just got done trimming off some of the older (brown or browning) leaves on my tomatoes yesterday. Part of it was just to tidy up the tomato garden, but I think it might improve air circulation since my plants too are getting awefully tall (but still are producing).
Also-- if our plants are tall but not producing as you think they should be, sounds like N-P-K ratio might be out of balance-- too much nitrogen, not enough phosphorus. I'm not a fan of Miracle Gro but you could try throwing some tomato-specific fertilizer at them if you still have time. What you're looking for is something like 15-30-15. At least something that has this ratio. Bone meal is a good source of organic phosphorus. That alone might promote increased fruiting.
Good luck!