You might can get by without one. Lots of people do. Just be aware, that's how the bearing preload is made. They almost always come out fine. Going in is where the issue arises. Although you can do it without the spreader, you risk breaking a preload shim, because without the spreader, you have tap the shims in to acheive beraing preload and those shims are very hard and brittle. Every salisbury type rear axle made is "supposed" use a case spreader to properly install all of the preload shims without damage. Plenty of people do it without them, but just know there's a risk.