Shocks and Coil springs

I have a 2005 z71 Avalanche. I am going to change out the shocks with Bilstein 5100. Online at Bilstein it is showing that the coils in the rear need to be replaced too. I am not adding a lift but I am going to turn the torsion bars for the front. What is everyones thought on replacing coils? Not sure if I need to replace or not

Also after I do the shocks I am adding new tires. I have had the truck since 47k miles. It has 118k now. I am thinking going just a little bigger tire. Anyone else have any suggestions on size. I will be using the 17" stock rims.

Ben 

Springs can go bad but it is usually a fatigue thing or they get permanent set where they do have a bit lesser free spring height. this applies to any metallic spring of any variety.

some are more prone to permanent set then others but this tends to happen if they are used with heavy loads.

TO check remove load and measure the free length, check for rust (reduction in material) and compare with original specs.

Stupid question but where might I find specs? I'm replace the shocks first to start there. I do haul a trailer loaded with 2k plus pounds on a regular basis so definitely want something that can with stand the load

GM I guess but my point is... springs tend to last a long time when you figure the rear ones are designed to support truck and all the payload.

How  many times have you driven with max payload?
Do you think it is sagging more then normal?

I suggest just sticking with what you have.

I wouldn't say that I have pulled it overloaded more than a hand full of times. I will give it a try with just new shocks and see if they need it after that!

Have you thought about air bags.

I have thought about them but wasnt sure how complicated they were

Do a google search "firestone air bags 4108" there's some videos on how to install them.