Vibration @ about 29MPH to about 32MPH
 

Seems to happen even if coasting at that speed in gear.
I got a TH350...

The truck's never run real smooth, but this shutter started happening pretty pronounced even more all of a sudden...

Re: Vibration @ about 29MPH to about 32MPH
 

If it's speed related, then it is likely either your driveshaft, something in the rear end or a rear wheel. I'd start by crawling under the truck with the transmission in neutral and the wheels chocked to check the u-joints (and carrier if it has one) for play. Also check the rear end pinion bearings for any play along with the transmission output yoke. For wheels, you could try swapping the fronts with the rears to see if the source of the vibration moves locations.

Has your truck been lowered?

Re: Vibration @ about 29MPH to about 32MPH
 

PJ gave sound advice above. Also bent or improperly mounted wheel and flat spots on tires will do some odd vibration. Dont overlook the center bearing on drive shaft. Rarely changed, they can be in pretty bad shape after 50 yrs.

Re: Vibration @ about 29MPH to about 32MPH
 

Yeah, the windows start to rattle at about 29MPH, can feel it in the body too, doesn't seem to effect the steering, I can feel vibration in the steering wheel but the truck still tracks straight. Up to about 32/33mph then the shake/shutter/vibration smoothies out mostly.

I've switched driver's side wheels front to back I'll do right-side today or tomorrow.
I'll check driveline while I have all my jacks and stands out, mainly checking for side to side/up and down movement and funky noises...rear wheel bearings....(?)

My truck's a longbed and it hasn't been lowered.

Thanks Bro!

Re: Vibration @ about 29MPH to about 32MPH
 

"Dont overlook the center bearing on drive shaft", I won't...Thanks!

Re: Vibration @ about 29MPH to about 32MPH
 

When it starts suddenly, my first guess is a thrown wheel weight