that means that the collar isn't sliding easily enough, which causes your fork to kick at an angle putting too much bending force for the hole to take. i'm surprised that the vacuum motor has enough force to break that though. try moving the collar by hand and see if it feels like it's binding up. remember, you may have to turn the axle shafts a little to get the splines lined up. if it moves freely, then i would suggest getting an after market steel fork instead of the stock one, if you haven't already. that would be where i would start trouble shooting. HTH
RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.
92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood