What is your truck, exactly? The shocks are different from 2WD to 4WD. Also how tall is your lift?
Regardless, are you set on the 5100s? If so, I'm sure that Bilstein makes some that will work, but you'll probably have to call them and ask a few questions to find the correct part numbers. Their contact information can be found at the link below.
http://www.bilsteinus.com/about-bilstein/contact/
If you're not set on the 5100s, the Bilstein HD is an excellent shock for these trucks. They came on Z71 package trucks from the factory. I run them on both of my trucks (2WD and 4WD) and I'm very happy with them. They're also cheaper than the 5100s. Check eshocks.com for the best prices, they're way cheaper than Summit or any other well known retailer last I checked.
Whatever you do, don't go Rancho. They're owned by Monroe and IMHO not a great product compared to the Bilsteins.
EDIT - Just did some digging around. By the sound of your post, I'm going to assume that you have a 6 inch lift on a 4X4 of some kind. Correct me if I'm wrong. I see what you mean about them not having a rear shock listed for a 6" lift. I would just call them and ask about the correct size of 5100 if I were you. I'm almost positive they make something that will work.