Well, you have several options.
Pull the rear end yoke off the old rear to see if it will work on the new one
or:
grap the driveshaft off the donor vehicle--if it isn't long enough for your sub, you can take it to a shop and have it extended, or just get one made for the new rear--that'll match your tranny.
SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4