Identifying rearend

So your car is a 74, and the third member drops out the front. This rear end is definitely not original to your car. 8 3/4 differentials in A bodies (darts) ended in 72. It is entirely possible that your car has had an earlier 8 3/4 bolted in, a dead give away would be if it has the smaller bolt pattern (5 on 4") wheels on the back, as no A body 8 3/4 had the 5 on 4.5" bolt pattern, only if it has been modified with different axles and brake assemblies. Another possibility is that someone put a ford 9" rear end in the car - this would mean 5 on 4.5" bolt pattern wheels, a front drop out (like the 8 3/4) but the rear surface is not a smooth round surface like the 8 3/4", it has a raised portion for ring gear clearance.

And FYI, Chrysler products NEVER had possis, or posis, or lockers or whatever. They had sure grip rear ends. Just different manufacturers names for the same thing.

Hope this helps, good luck!