1969 Dodge Dart issue with installing axles after thrust block removal.

if you turned the axle shaft, when the other axle is removed, the splines on the empty side can spin relative to each other becoming mis-aligned. You can disassemble everything and loosen the bolts holding the SG together until you can reinsert the axles fully then retighten the SG fasteners. OR like was suggested you can use a block of wood against the studs like a lever and rap the wood lever, hoping to create just enough slippage in the clutches to allow the side gear splines to align and the axle to fully seat. Repeat as necessary for the other side. Once you have them aligned, reassemble third member into housing, carefully insert BOTH axles before you start spinning any axle shaft to tighten the axle shaft retaining bolts. Enjoy.