My first guess is the rear axle and spring connection. But only if you have a torque tube.
Be sure you have a good and functioning rubber bushing on each side.
page 4-61 in the 48-53 service manual.
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-