If I am not mistaken, the 380 has a 3.46 different ratio and the 300sd is definitely a 3.07 ratio. The 617 will be turning some high RPM's at a higher speed with the 380 dif.
As far as your issue, the best way is to swap the yoke out. Either at the trans end to use the 380 driveshaft or the diff to use the 300sd shaft. Swapping out the yoke on the diff is a bit more trickier because you need to pay attention to the rotational torque. Do a search on this subject. Plenty of info on that here.