The waste gate probably has a build up of rust around the shaft and it is getting stuck open, dumping the potential boost. Any competent mechanic that knows turbos could pull it apart and free it up. Turbos are rebuilt everyday and you could even find rebuild kits on the Web and do it yourself. The problem is there are not many real mechanics around anymore so they'd rather swap out the whole unit. Unfortunately this is at the customers expense. Unless there was complete bearing failure or cracked housing, they're rebuildable.
I understand the OP is covered under a warranty but I thought I'd post for others too, or incase the warranty doesn't cover it.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS