I have searched and searched and searched, but have yet to find the same set of circumstances.
I hardly ever run on the freeway, and last weekend I tried. From 68 to 85 my truck tries to shake the cab off. Hmm.
In the past week I have done the following:
Rear drive shaft ujoints
New carrier bearing
Shimmed carrier bearing support
Rotated tires
Balanced tires

I still have the shake. Seats, mirrors, dash, all shake, but nothing thru column.
I also have a suspension hop now, but that was probly there before, just never noticed, and it is more than likely shocks.

I have been hearing a squeak sound as I back out of the driveway in the morning and the chassis flexes, and when I pull into shop and flex occurs. The squeak sounds like it is behind me, and does not get louder as I try to listen with my head hanging out of the truck.

Could it be a loose body bolt? I am going to retorque the cab bolts tonite after work and check the bushings.
If it is loose body mount bolts, could that cause my freeway shake?

Any ideas will be appreciated, I am out of hair to pull out.

 

Did you check for belt separation on tires?

 

Hmm, maybe, the front lh mount being so loose that it almost fell off in my hand, was it. I couldnt take it anymore and ran out and checked the mount bolts while I am "working", and found that one.
Lh rear was about a turn and a half loose as well, but no where near as bad as the front one.
I really hope that is it, I got it running great, now I gotta get this figured out.

 

have to agree with ^^ - I've learned to always check tires out first.

Expensively I learned that lesson....but I did. lol.

good luck on your issue.

 

Update:
Tightened body mount, that stopped the odd noises and the hop.
Freeway shake is still there.
Tires have been to 3 tire shops, for balance and check. Never explicitly asked about belt separation, but wouldnt the tire have a bulge or something?

But, the weird thing is, it doesnt always shake. on cruise on the freeway, the shake starts, fades out, and fades back in. Over, over, and over again. It is cyclical. Maybe that will help someone with more experience help me figure it out, because I have exhausted what I know on the subject.

I am going to throw my 351M I have waiting to go into my sons truck in the back of the truck today to see if that helps some.

Again, thank you for any help that can be offered.

 

Btw, the tires are Nitto Terra Grapplers with about 7k on them

 

Are the tires showing any sign of cupping?

 

No cupping, no chopping, no uneven wear.
If I had the money, I would buy another set of tires and rims for winter, but I dont.
Is there any way I can have the tires for checked for belt separation?