Rtm

Hi guys, I have a 99 Forester I brought off a friend few months ago we changed the gearbox due to crunching 1st gear and installed a new clutch, pressure plate, thrust bearing, and fly wheel and now it's slipping like the clutch is slipping but it's not the clutch. It slips bad in 1st 2nd and 3Rd. I also noticed that I can spin the driveshaft with both back wheels still on the ground so is this diff stuffed or are Subaru diff suppose to be like this? Also Been wondering if the back diff is stuffed would it cause the viscous coupler to slip and drive like a slipping clutch? Any help would be much appreciated thanks