Aloha, my car shakes or rumbles,(not in the steering wheel) when driving between 40-45 mph. You can feel it in the floor by my feet and car seat. It feels like when you go over road strips or rumble strips. I’ve taken it to few mechanics and no one knows what it is. My tires have been balanced as they say, a couple times. It stopped for a few days after rebalancing but now it doing it again just not as bad. Any ideas. I would love to finally get an answer so I know what to fix.
Mahalo
You need different mechanics or–even better–you need take your vehicle to a well-reputed independent transmission shop.
Why do I say that? I say that because you have perfectly described a problem with your transmission’s Torque Converter Lockup mechanism.
Don’t go to a chain-run trans shop (Mr. Transmission, Cottman, Lee Myles or–God forbid–AAMCO), unless you want to be sold a new/rebuilt transmission when a less-major repair is all that is needed.
Oooh, that sound like a big job. Well I kinda new to my area so Im not sure who’s reputable but non of those are near me. 
I’m not sure how bad the problem is but if I don’t get it fixed soon, can that become a very bad situation?
To find out if it’s the lock-up torque converter causing the shake, when the shake starts, slightly step on the brake pedal while maintaining speed.
This will cause the lock-up torque converter to disengage.
If the shake stops, it’s the lock-up torque converter causing the shake. If the shake doesn’t stop, look at steering/suspension components.
Tester
On some transmissions, the solenoid that controls that mechanism is readily accessible on the outside of the transmission case. However, I don’t know if that is the case with this Honda’s transmission.