I need to replace one of the front sway bar linkages and have a few questions.

1. Is it under tensions (do I need to block up the ball joint)
2. Is it pressed on or will they just pull off when the nut is removed

The last sway bar linkage I did on my jag was a while ago and if I remember I had to block up the ball joint and the linkage just pulled off when the nut was removed.

 

Just remove the 2 nuts and replace it. More than likely it will stay in place

 

If you raise the front end and support it on jackstands on each side, there will be little tension on the fittings. If you jack up just the side you are working on, the tension will be prohibitive.
The nut at the upper end of the link where it connects to the strut was very tight on my car; tighter even than the two big ones that hold the strut to the knuckle. I highly suggest an impact wrench. If you don't have one, a long breaker bar and a thin wrench for the flats on the backside of the end link may be necessary. Good luck.