The front shaft has a certain phase which can be lost IF you pulled the yoke slip joint apart. Since the front has a steeper angle than the rear, those U joints are usually 1st to fail. Be sure to lube them every 5,000 or less.
Check the LR service manual to get the correct phase of the U joints, I think it supposed to be 90 degrees off from the fixed end yoke U joint, but I'm not 100% positive. It's always prudent to mark the slip joint & main shaft in case you happen to pull them apart.