2000 Discovery II with 89k miles

I am preparing to purchase a spring conversion kit to replace rear air bags. How important is it to replace shocks?

I believe this kit http://www.roverparts.com/Parts/9268D2ABP.cfm?bc=ab has everything I need.

 

Very important, especially if they are still factory.
How many miles on your truck? Specifically what springs are you wanting to install, the wrong one can lift the truck too high or not high enough.
By the way, why are you replacing the SLS instead of fixing it?

 

89k miles on the vehicle

http://www.roverparts.com/Parts/9268D2ABP.cfm?bc=ab are the springs I am thinking of buying.

SLS seems too unreliable. They are deflated. Pump not working at all. Very disappointing for a vehicle with only 89k miles. One simple fix (springs) no more issues. Well, plenty more issues, it is a Landrover, but just no more with the air bags.

 

Should have asked first, I could have walked you thru the SLS issue.
At your miles, you would be a lot better off installing a Old Man Emu kit front and rear along with the adapter kit to convert the rear mounts to coils. Doing this will give you a much better ride, a more level and equal ride cause your current front springs, steering stabilizer and all 4 shocks have long since warn out.