Except the driveshaft would be locked in fixed linear planes (up/down - left/right) which would not accommodate the inevitable body flex and other movements that certainly happen that would otherwise deflect/bind the driveshaft.I never gave IRS driveshafts much thought, but from a theoretical perspective both the transmission and rear end are locked in place and you should be able to use a solid shaft, no u-joints. I assume the little bit of play in mounts, etc make the joints necessary.