110 rear half shaft keeps popping cap?

87 110 County, Friday pm i saw my rear PS plastic cap was split, letting grease/oil out. I have recently replaced both drive flanges as knackered, so both sides have new caps. rear salisbury type axle with drum brakes.
after reading up on here thought it might be breather blocked and pressure in shaft or somet? so i took off breather which was blocked, drilled it clear and cleaned. put back on and tested pipe. all ok. new cap on.
next day - another new split cap - this time noticed the half shaft sticking out a bit so it must have been the shaft that split the cap/s?!
another search here and someone asked if the shaft is held in only by the cap - the answer was "yes" - is that true?
why is mine splitting - i only have one more cap left! anything i can check/do?
thanks

shafts held in place by the length of its splines and drive member,are ends of shaft worn were splines finish and round shaft starts

don't know, i'll have to strip it again. don't remember it being bad when i changed the drive flange - it was the flange that had no teeth left and gave rotational play.
thing is - that was over a month ago and been fine til this week?
anything in the diff end to retain - shall i strip, pull shaft out and pop back in different place?
also, i have the stubby caps and noticed some people have taller caps - should i use taller caps to allow the shaft to come out a bit - i presume it can also slide back in under drive?!!

There's nothing that keeps the shaft in place at the diff. As James says it's just the splines ending and pushing against the drive flange.

Very badly worn or damaged splines on the halfshaft will wear the flange quickly allowing more movement.

You sure it's not just a blocked axle breather?

that was the first thing i checked and it was blocked but now isn't but still broke second cap

Some of the cheap ones I have found to split in the cold weather. All the oem ones i have left are ok.

Cheap ones seem to be if a poor quality El cheapo plastic

I had a bit of excessive end float on my halfshafts a few years ago. So much so that one of them floated all the way out and I lost drive to one wheel https://www.landyzone.co.uk/land-rover/driveshaft-end-float-is-this-normal.198564/. It tends to happen as they get a bit worn. Initially, I got some of those drive flanges with screw on caps so they couldn't come off, and then eventually I got new halfshafts. Seems OK now.

I wonder if the shaft is moving in and out and fretting the cap? Mine has shims behind the circlip and moves barely 1 mm.
+1 on the cheapy caps, the good ones are like a semi soft rubber rather than hard plastic. If you pop them in a jug of boiled water for a min they go on easy, no need to batter them on.
Like others have said though, it is possible the plain unsplined part of the shaft is worn allowing more outward travel or the flanges are badly made maybe?