4wd Issues

[Lifted94XLT]

Here Lately I have been having some issues with the 4wd in my 04 250. it will always shift into 4wd fine, but the problem is at the front axle. Sometimes it will engage, and sometimes it won't..... and when it does engage the front end, it will jump in and out of locked, if that makes sense..... My Drivers side hub assembly was new last year, and my Pass. side is new as of about a month and a half ago...... so I'm down to it either being a hub (on side or the other or both) or possibly the vacuum lines that run down to each hub assembly to engage them...... I know others have had this problem before.... oh yeah, if I lock in the hubs manually, it seems to lock in and stay in..... so now possible causes is down to just the vacuum lines..... is there anyway to test this?? also, does anyone have the part numbers for these two lines so I can check prices for new ones (couldn't find them on powerstrokeshop.com)

Thanks for any help y'all may be able to throw my way.

--Ryan