What if anything does the leveling kit do? Does the ride improve..any roll? Are there any mechanical concerns ..driveshaft etc..thanks
When people refer to the Bilstein 5100's (and comparable products from other manufacturers) as a "Leveling Kit" there's really not much to them mechanically. It's a new set of struts with an adjustable lower spring perch that lets you choose the ride height by changing the distance from the lower spring perch to the pivot bolt on the lower control arm.
Since trucks are made to tow and haul it is typical for the front to sit a little lower than the rear to ensure predictable handling while loaded, but some people don't like the way that looks when they're unloaded. When people level a truck that means lifting the front of the truck to match the height of the rear. If you lift both ends that's just a lift.. though it's typical to lift the front a little more than the rear to produce a level final result.
Here's the executive summary:
There is no real mechanical concern. Lifting the front over 2" you are strongly advised to also buy lift-specific upper control arms. Lifting over ~2" can result in a "Pogo Stick" ride some people don't like and others don't notice.
Nerdy wall of text to follow:
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Because it keeps your stock range of motion and doesn't mess with any of the stock fixtures there is no change from stock mechanical reliability. Total suspension range of motion stays pretty much the same, maybe a quarter inch different if you want to split hairs.
There is a "BUT"...
The "BUT" if you select a ride height more than about 2" above the stock ride height you may have trouble getting a good alignment using the stock upper control arms. This is because the upper and lower control arms are different lengths from each other. When they are both level (parallel to the ground and each other) that's fine but as you change the ride height they move out of parallel and the camber angle of the wheel changes. There are alignment cams on the truck to correct for this, along with correcting for minor collision damage, manufacturing inconsistencies etc. like any vehicle - but they have a very limited range of motion and when you raise these trucks above ~2 inches or so with otherwise stock parts you can reach the limits of those cams.
Regarding ride quality every brand of leveling strut will have its own secret sauce. Bilstein usually builds their 2" monotubes with a digressive damping profile - in English that means firm on small bumps and less firm on big bumps. That means they set into corners more quickly and feel sportier, but it also means you feel the roughness of the road more. Whether you like that or not is entirely personal.
Because the lift/leveling struts don't change the factory range of motion but only where your truck normally sits within that range, there is also a trade-off between "up" travel and "down" travel in the suspension. Your suspension can move up and down roughly 9 inches total so if it starts out with 4.5" up and 4.5" down, if you lift the truck 2.8" now your tire can only move down 1.7" from its usual position relative to the body. That means you might find the truck falls into potholes more, falls off the far side of speed bumps more.. just like other aspects of ride quality that affects different people differently some people can't stand it, other people don't notice it.
Lastly there's minimum preload on the spring. Because these leveling struts use a pretty much stock length strut body and just change the location of the lower spring perch along the strut body the more you move it one way or the other the more you affect the neutral preload on the spring. When the truck is at rest or moving on level ground this specific factor doesn't affect ride because the springs are linear. Their static length while loaded is the same no matter how much preload they have (as long as preload is less than static load) . But when you take the load off the spring by lifting a tire entirely off the ground, the preload becomes the minimum amount of weight necessary to make the suspension move and do its job. It might be hard to imagine what that feels like but most people who have noticed it seem to agree on one description: Like a pogo stick.
It's hard to say authoritatively what the correct setup is, everyone has different expectations and values and there are a lot of right ways to do it. My own truck has been through several iterations of lift components and settings, affordable because I can do my own work but if I was paying labor I'd definitely be happier if I had got it exactly right the first time