Birfield joints are normally fastened to the inner axle with a small circlip. This clip is a devil to install, and even harder to remove.

Installation requires ten or twenty spare fingers, and a sailor's vocabulary. Basically, you put the clip onto the inner axle, then slide the inner axle into a greasy Birfield while trying to keep the slippery little clip compressed enough to stay in the groove -- all so that you can slide the axle and clip into the Birfield. When the clip slides in past inner bearing race, the clip expands and retains the axle in the Birfield.

Removal, once you get the inner axle and Birfield out of the axle, is even more entertaining. You can uses a CV-joint splitting tool, or beat the hell out of the joint... I 've had some success repeatedly dropping the axle and Birfield assembly into a pipe that would fit the axle but not the Birfield. All of these techniques basically do the same thing, force the Birfield to cut through that circlip.

Tired of fussing with those clips, Marlin Czajkowski, of Marlin Crawler fame, started thinking. The clip isn't intended to keep the Birfield from pulling off the axle and heading outboard from the centerline, because the Birfield's stub is stablized and constrained by the hub and spindle. The only real purpose for this clip is to prevent the axle from sliding out of the Birfield, toward the center, and into the differential. Normally, the axle floats inboard and outboard a small amount, constrained by the clip at one end, and a bigger c-clip on the axle.

Look closely - there's a tack weld on the end of the spline.

Marlin tacks three places around the axle, all just over one and a half inches outboard of the inner end of each axle where it slips into the differential (red dots in the diagram). Any old welder will work: arc, MIG, or TIG. Just tack it lightly, you want to deposit a small amount of rod but minimize heat transfer. When you reassemble, you can leave out the end circlip (blue arrow in the diagram), but be sure to include the square-shouldered clip (green arrows in the diagram).