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Make sure that the washers are on the back of the gears and have not slipped out of place.
Rotate the center housing so you can insert both gears like in the pic below.
Engage the differential pinion gears opposite the differential side gears.
Use the shaft at this point to make sure they are lined up if you have to.
Rotate the differential pinion gears to align the differential pinion shaft bore.
You will have to also rotate the center housing so you can insert pin at this point. I would have the housing so the part where the bolt goes thru the center pin is at the bottom. Then after you get the gears in and aligned rotate the center housing a little so you can insert pin from the bottom.
Insert the differential pinion shaft
Ok you have a limited slip or traction lock rear axle so that presents a whole new problem because the clutches are pushing out on the axle gears..
You will still have to put both gears in like the second pic I sent you,put the center pin thru them to make sure they are lined up then remove the pin.
Now rotate the center housing so both gears walk around the axle gears at the same time until they line up.
It may take 3 hands to do this,2 to hold the gears and one to turn the center housing