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Sounds like the used transmission has the wrong ratio front diff in comparison to the rear diff.
The easiest way is to get the transmission serial number, and check with a Subaru dealer.
They can decode what ratio is inside if it is still stock.
Give them your VIN as well and they'll tell you your rear diff ratio.
Don't drive it until you get this sorted.
Hope this helps.
I can't tell differences for you from trans serial numbers.
I can only suggest asking Subaru how many different diff ratios are available for your Model/year.
The numbers mean nothing if somebody else has rebuilt the donor trans with a different gear set.
10-15 mph is a big difference, even at 55mph.
Have you asked Subaru about this?