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Glad to find this thread, mine have a lot of endplay, too. We noticed it when we changed the bearing seals a while ago. It doesn't seem to be getting any worse, and I've got 106k on it now, but it sure is alarming the first time you grab an axle and pull on it.
At least some of the end play comes from the clutches in the Traction-Lok assembly wearing out. Assuming you have a TL rear (all AWD vans do), you might check to see if the diff is still locking: Jack up just one side of the rear, and try to turn the wheel. Ford specs about 40 lb-ft of break-away torque.
There is nothing that solidly holds the axle shafts into the housing. On the inner end, the cross shaft of the diff carrier form the inner limit for the motion of the axles, and the C-clip (horseshoe clip) pushing against the clutch pack and side gears form the outer limits. As the clutches wear, there is more slop for the axle shafts to move in and out on.
My 2WD doesn't have traction lock, but it still has end plays.
Hmmm, I've never opened up an open 8.8 diff before, so I don't know what it looks like inside. I'm sure it also uses a C-clip on the inner end of the axle shafts to keep from sliding out, and the cross shaft is the inner limit, so there could still be gaps due to production tolerances. How much end play do you have?
I have 7.5" differential for both my 2WD and my 4WD. The 4WD has traction lock while the 2WD does not. But aside from the clutch packs, the two differentials are identical, and they both use the C-clip retainers, hence the same end-play.
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