It can be done in the truck.
You will need a dial indicator, and a mount that bolts to the axle, or a magnet mount base for the indicator. A Helms or = service manual will be allot of help. You should install new side seals, there is a tool that installs them with ease, its basically a jacking bolt. You tighten it and press the new seals in from against the other side of the inside of the diff. When you buy your gears ask them to include detailed instructions. You will need to crush the crush sleeve. It takes some brute force, and ignorance, a quality breaker bar and a long pipe, or an impact gun with some serious umph.
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