From my understanding the JTLV isn't a "replacement" for the HMMWV and the Army will continue to use HMMWV's for "truck"-type jobs... things like delivering troops and materiel around inside compounds or between CONUS bases. The JTLV will be used more like an APC or scout car. It's not intended to deliver T-rats to infantry in the field, or to carry ammunition. It's meant to take part in combat--and it's way better suited to being a gun truck, or a TOW vehicle than the HMMWV. Which was never designed for that and which did its best in up-armored variants but wasn't really satisfactory in that role.
If you look at what Oshkosh produced, it's pretty clear that it's not really a "truck" in the same sense as a HMMWV. It's a combat vehicle. For that purpose, it's vastly superior. But for "truck"-type jobs, the HMMWV will probably stay in use for the foreseeable future.