I've done a couple on the 900-series (1st Gen IFS) pickups, and a handfull of ATVs. In my experience they're all very similar, so hopefully this info will apply/help you. It does sound like a birfield going out to me. Either that or a wheel bearing, but considering the boot has failed it's probably the axle. Grab it and shake it around, should be able to see where the slop is. Here's how you'd do it on my junk:
Remove axle-stub nut (It's a big-sucker, like 1 3/8" on a truck, 7/8" on my ATV. If it's frontwheel they're usually even bigger. Most parts stores carry these).
Remove splined section of stub shaft from knuckle/spindle. This really requires a pusher/puller that mounts to the WMS with the lug nuts (rented at parts store, I bought mine at FATS for like $90). Sometimes it can be done with a claw extractor, and I may have noticed a cheapy version that would work at Harbor Freight. Some guys will also use a drift and a BFH, but I wasn't able to do my ATV axle with a 5lb sledge...guess it might work well in drier climates (less seizing on the splines).
On my trucks, you need to remove the anti-sway endlink to move the axle around in between the A-Arms. My prayers are with you that you don't have to get into any balljoints or TREs to do yours. On my ATV, you have to pop out a balljoint unless.....I fingered out that I could just remove the two bolts on the upper A-Arm that go through the bushings. Before you get too destructive (if you even have to), see if there are any fasteners you can fiddle with instead. If you do mess with the A-arms, keep in mind that your caster is usually adjusted here with shims. Keep track of shims, or consider replacing those bushings (probably cheap) and getting an alignment afterward.
On my trucks, the axle mounts the to center section with a flange like some drivelines use. I believe it was (8) 9/16" bolts. Most ATVs I've worked on have a splined stub that is inserted into the center section as opposed to the aforementioned flange-style. These can be weird (splined versions), as there's usually a seal on the stub. A sharp jerk usually pops them out, but I personally shy away from using force unless I know it's the proper method. That said, the first shaft usually has such a vacuum behind it that it feels like it's bolted in there somehow...On second thought, you'll have the new axle with you. You should be able to see how it fits in there. When I did these the first time, I was waiting on the replacement in the mail so I didn't have any reference.
From here on it's pretty much just reverse order. If you remember one thing from here, make it this: Use anti-seize on the splines going into the hub/spindle. There is no good argument not to, and it will make future replacement so much easier. You won't need the pusher/puller if those splines are properly lubed. I can literally push my stubshafts out with my finger after I remove the nut.
