If the freehub body itself is loose, remove the axle and see if the 10-mm hollow bolt that fastens it in place has gotten loose. If so torque it back to spec. . If the cassette is loose on the freehub body, retorque the lockring and, if it'a a 10-speed cassette, did you forget the 1 mm spacer that should be behind it? A replacement freehub body is easy to install but be sure your isn't just loose.
I also have a 7700-series freehub with 55,000 miles on it and it works perfectly so they are pretty durable hubs.