Weird Feel From Underneath Floorboard

I'm having this issue where whenever I get above about 40 mph, I have this weird feeling from underneath my drivers floorboard. It doesn't happen at any speed below 40mph. The best way I can describe it is like my front drive shaft feel like its doing a bit of vibrating. Today I removed my front DS to see if it cured and it seemed to improve it a little. But I can still feel it. Its not terrible but Its only on a certain spot on my floorboard which is right in the center more towards the the front of my seat. I can also feel whenever im braking kinda of harsh from high speeds. Im stumped at to what it could be. Only thing off the top of my head that I can think of could possibly be front tires need balanced, something with the brakes, or i just need an SYE even more now. The last thing I did to my jeep was re gear it throw some long arms on it, but it had been about 3 weeks since I've done that and this has started up about a week ago i'd say.

Switch the front tires to the rear and see if it changes, that can help rule out the tires.

How tall are you now? I know you've got a 97+ XJ now and they're more prone to vibes than your 88 was. Vibes and sounds have a funny way of traveling through the vehicle.

Whats up Tim?
Im sitting at about 5" now. I have come to the conclusion of 3 things it could be.
1. Caster needs adjusting. (Recently installed Long arms)
2. Front left tire needs to be balanced or replaced
3. Need to do my HnT. I Currently have a TC drop and shimmed 6 degrees, which put my pinion angle pretty close to where it should be, but not close enough I guess.

Gonna rule out number 2 tomorrow morning.

It honestly sounds like a tire balance issue at the moment. Did a balance and rotation solve the problem?