I don't believe that a full float Dana 44 was ever made by the factory, so it's likely that you've got a conversion kit. If that's the case, hit up this guy for guidance:
http://hermtheoverdriveguy.com/?page_id=1611.
How sure are you that it's a Dana 44? I went back a took a look at your old posts (
viewtopic.php?f=7&t=6861). The ad from when you bought your truck is long gone, but there's a post of mine positively identifying the rear end as a 9". As Kirk mentioned, Warn sold a kit for years that converted the 9" to full float using Dana 44 parts. I'm guessing that's what you have.
Either way, it sounds like you've got some hub parts sticking. I suspect that water, time, and grit have combined to seize up the locking mechanism in the hubs. There's some parts inside that slide back and forth with a spring to help actuate them when you turn the hub to the locked position. If the stiction in the parts is higher than the force provided by the spring they don't lock. It could be that they're wiggling loose when being driven, and that the spring can't keep them in place. It's a simple fix to pull the hub apart and service it. It's also a great opportunity to repack or replace your rear wheel bearings, which are probably in equally bad shape. If nothing else, a full servicing of the parts is easy, never hurts, and would eliminate the outer parts of the axle as the source of the problem. It's pretty unlikely that the locker, gears or other parts inside the center chunk of the axle are the issue.