Jack up the truck and support it very well in front of the rear springs.
Cut the U-bolts, roll out the old axle and roll in the new one. Use new U-bolts and nuts.
This can be done by one person if necessary.

•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-