I need to replace a cv boot on rt front '98 4wd.
Any suggestions on how to separate the axle at the first knuckle?

 

If you mean to change the boot with the axle in the truck, don't. Pull the axle so you can work with it better. There is a snap ring that holds the outer joint to the axle shaft which is buried under the boot and a ton of grease. To get it apart, you just beat it off. (heh) with the axle in, you may inadvertently separate the inner joint without realizing it- that's a real mess once you're trying to reassemble.

 

If your CV boot is torn your better off just replacing your entire axle since containment's already got in the boot the cv joints days are very numbered

 

Amen.