Hello, any time I have replaced a boot it is much easier to replace the entire axle.
If you only replace the one boot, the other is just as old and will tear. You already have to take the wheel, hub etc apart and get the front end aligned. You are there. Save yourself some time.
Also, if you have ever tried to replace the boot only on the old axle, you have to have a special boot clamp tool and it can be challenging putting it all together. Also, many times there are parts on the axle that should be replace that when you replace the boot only, you miss that part.
Last, many times the expense for the entire axle is not much and well worth the cost of just the boots and what you go through to remove and replace just the boots.
Hope this helps.