4wd/2wd shifter fork (front axle)

[hiduke89]

With the housing for the fork off the front axle, with vaccum lines attached, the fork moves both ways fine when shifting from 4WD to 2WD with shift lever in the vehicle, but when remounted to the axle the 4x4 won't engage. There is about 3/8 to a half inch slop in the fork on the rod it is attached to....Could this be causing it to not engage into 4 wheel drive?
When put into 4wd manually (removing the cover, and sliding the ring towards the differential, remounting the fork and housing on to the axle) Taking it out for a test drive, it is engaged in 4wd.