Anybody done a 2/4 drop on a lifetime warranty truck? I want to do one but was wondering if there might be a problem down the road with a U-joint or driveshaft? I know they would have to prove the drop caused the problem, just wondering if anyone has experienced any?

call your dealer, talk to the service manager (anonymously), and just ask him.
the drop "should" re-align the rear end to the correct (-2degree) angle, and would actually help the u-joints, as it would be more in-line with the drive shaft.