Rear pinion flange won’t connect to driveshaft

Hello I’m having trouble w this could yall help, I have a 2006 Mustang GT and I recently had to replace the whole differential, I got the same gear ratio and all that but the pinion flange on the new used differential seems to be a aftermarket one because it’s short and the driveshaft can’t reach it to connect it, I’ve also tried to swap it out w the stock one but it has a different spline count. So at this moment I’m stuck and don’t know what to do I’ll gladly appreciate some help and advice. Thank you.