I can't tell under the dirtball you have whether there's a u-joint or not, so advice on safe-ness is difficult! The force required to slide the halfshaft pieces back & forth on the splines is normal but be advised that you can completely separate the halves, and reassembling them ONE spline off will cause your car to shake like a wet dog! Mark the two halves somehow if you decide to pull them apart for cleaning. Good luck.
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