Are you sure you got it assembled correctly? The rebuild kit I used had a couple different spacers. One spacer had a longer stem which might restrict compression of he spring.
You should have pre-greased the parts on assembly, but even grease wouldn't help if it's overtightened (especially if the balls are worn).
The service manual says:
"...tighten plug snugly to remove all end play of ball.
2. Back off screw plug one complete turn plus amount necessary to insert cotter pin and lock adjustment.
Ball joints must be tight enough to prevent end play and yet loose enough to allow free movement.
Be sure sockets have ample lubrication."
Granted, "snugly" is a pretty vague value, but they have to be able to move.
