Shaking at higher speeds
 

First off, I've seen many threads about this, but can't seem to find something specifically seeming to be exactly how I'd describe it. The speed range is between 100-120km or so when I feel it. The car has vibration sort of throughout, but there's a really noticeable shaking right under the driver seat (not sure about passenger, and yes the bolts of the seats are secure haha). It comes and goes in that speed range, will be heavy then It calms down and starts back up again. From what I've gathered from reading around the forum, possibly ujoints? Truly not sure, anybody ever have the same sort of thing and can tell me what they did to fix it? Doesn't seem to hinder anything, just a very annoying shake, and not sure whats causing it. New to mechanical side of things and rotary-wise, definitely still learning so be easy haha

i want to say drive shaft because i felt the vibrations every time i go into neutral from like 4th gear or 5th gear at high speed. had to replaced mine a couple of years ago on my old fc. when i first got the fc from los angeles, i drove 400 miles home with that problem. didn't really know what the problem was, but thought it was the differentials. i then drove it around for 2 months before the ujoints gave out. luckily i was down the street from my house on my way home. had a hunch that it was the drive shaft and replaced it. ran like new afterwards.

U Joints are the number one suspect, but as it comes and goes, take a look at the motor mounts.

Bad lower Ball joints can show at speed.
Also a flat spotted tire can make the car jump a bit,like vibration.
Bad U joint on Driveshaft as mentioned.
Unbalanced tire,bad wheel.
bad mounts on drivetrain.,

That's a little list to go through.

Thanks guys! I'm really busy with school winding down here. So updates on the situation will be heavily delayed, but once I find the exact culprit, I'll post about it.

the diagnosis steps are as follows.

1. since its free, step one is to hop under the car and look for loose/worn tie rod ends, ball joints, wheel bearings, any play in the drive shafts, bad mounts, etc. repair as needed.

2. since tires are a likely culprit, a rotation and balance is a good idea.

3. if its STILL there. you're looking for bent wheels, out of round tires, out of balance driveshaft. bad water pump (i've had a couple with bad bearings, its surprisingly noisy, yet don't leak water).

thank ya kindly, I'll do that then when possible

if you feel the vibration in the seat you can usually rule out the ball joints, front tires, ect. because you would feel it more in the steering wheel.

I've got my money on Ujoint or rear tire issue.

i was cursing with somebody from the forum a couple days ago and he said my rear tires were bouncing around like hell