2015 with thumping/ bad vibration around 30-45mpH UPDATED FIXED!!

First real gremlin in my 2015 platinum with 115K. Preface this by saying I’ve had numerous F150s and work trucks with the death wobble occurring. This is different.
At first, I thought it was in alignment or balance tire issue. Brought it in for another alignment. It seemed to fix it for a short time. Then, after parking on a slight incline in setting the parking brake, I release the brake polled away going around 35 miles an hour, and i started really feeling something like almost a flat tire and a thumping under the drivers seat. After parking at and doing some shopping, I drove it. It was fine. Later that night I wanted to see if I could replicate it. Went for a short ride and started doing it again worse than ever to the point where I almost had to pull over. It’s in the shop now, so I’m waiting for them to try to diagnose it. Anyone with something similar please chime in.

 

Does it feel sloppy like moving side to side when driving or just an audible thump and bouncing? Could be snow stuck I wheel or even a wheel weight that jumped off...I hope it's an easy fix!

 

No sloppy steering. Hasn’t snowed in Massachusetts for a while. Definitely something loose or not right—very intermittent. But concerning like almost like a flat tire or something banging underneath the middle of the truck drivers area.
UPDATE:
Previously was taking my truck to Town Fair Tire where they can’t do an actual four-wheel-drive alignment. There was always one part of the paperwork alignment that was red. They can only do the toe apparently. Great guys at my local Townfair, but are limited with equipment Currently at the dealership for a proper four-wheel-drive alignment will update how it rides if the issue comes back.

 

Upon further inspection by the dealership, they told me, my E brake was sticking, which could’ve definitely caused the shutter, as well as the front rotors being warped slightly
Truck is currently at an independent garage I trust and I’ll wait until I get it back to see if it resolves the issue once this is fixed

 

More in the seat. Initially, it almost felt like a flat tire driving down the road then started to shake- felt it under the seats and directly under the vehicle. The only two things I did that day was go through a touchless car wash and use my E break just before this happened parking on a hill, then taking off.

 

I am not sure. After the dealer unstuck the E brake and did a proper four-wheel-drive alignment I didn’t notice any abnormality driving. On a sidenote, I guess my local Town Fair Tire doesn’t do a legit four-wheel-drive alignment. They only adjust the toe But like I mentioned it’s in the shop hopefully get it back this Monday. I’ll ask my mechanic to check it out before I pick it up.

 

I had the e brake cable stick on my truck several winters ago, as well as another vehicle that we purchased used several years ago. It caused the same strange thumping that eventually went away but was quite disconcerting until it did. When I read your first post, that's exactly what went through my mind so it is certainly a legitimate solution. The dealership replaced the brake cable on my truck as they found corrosion in it and I haven't had an issue since. I've also made it a habit to use my parking brake much more frequently; I apply it whenever I'm on even a slight incline, or whenever I park with a trailer hitched up regardless of the incline. That practice seems to have served me well.

 

So you hit the nail on the head with the E brake. That’s exactly what the culprit was. I engaged it and disengaged it before it started to vibrate when I went down the road afterwards. I guess it’s just a common thing. The mechanic showed me the part that he replaced. It’s a hinge like device that connects to the E brake under the truck. I hope this helps somebody in the future.