I am starting to think your tires are not your problem. Not sure if this is your problem, but I have had the driveshaft center bearing mounting bolts loosen and cause a bad vibration, bad enough that I pulled over about the instant it happened to see what it was.
On another truck I had the rubber part of the bearing fall out, it had a little vibration, but not really bad. In this case I was able to still drive the truck home about 40 miles with no problems, I bought the truck just for the engine, so I was not concerned with the damage I may have caused to the driveshaft, so long as it got it's self home...

I ran 8X19.5 bias tires on my 1 ton, after about 8,000 miles I put radials on the rear only.

I did notice that they rode much smoother in general compared to the 7.50X17 trues I was running. A hole or bump in the road felt much softer than with the old 17" tires.

You can find used or new 19.5 wheels to fit your 3/4 ton, and I highly recommend the upgrade to 8R19.5 tires for a working truck. The wheels will clear the tie rods.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-