How far north? There are always places to go almost anywhere. Part of my family is from the Lüneburger Heide, and there are several places to hike. Königsberg near Uelzen has some nice trails. Don't expect wilderness, however.
How's your German? Almost all the towns seem to have their own webpages. Fallingbostel's website includes suggestions for hikes and biking. If you don't speak German, I can translate for you. Email me privately on this and I'll be glad to help. The locals are usually also glad to point out good places to go.
From what part of MN are you? I'm originally from the Fargo area, and nowadays i live about 60 miles from Duesseldorf, in a town called Nijmegen. Maybe we can meet up and go for a hike or so.
Anyway, Moni is absolutely right: Wilderness does NOT exist in Northern Germany, and Southern Germany doesn't have much of it either, maybe an alp or two, and some forest in the Bohemian Highlands... There are a lotta nice hikes there, though, and if you're going to the north, don't miss Brocken, which is the highest point in the Harz range, and has the worst weather in Germany, because it sticks out 1500 foot above anything else...
Moni, do you mean Uelsen near Emlicheim???
I have as much authority as the Pope. I just don't have as many people who believe it.