99 4X4 problem

[06DieforDiesel]

ok so my dad has a 99 f350 (gas of course) and its having 4X4 problems. plain and simple it seems to have gone out. it does have shift-on-the-fly but when he turns the dial inside the cab nothing happens, it doesnt go into 4 wheel drive. also the 4X4 light comes on in the dash when the dial is turned just not engaging. we have been manually locking in the hubs then turning the switch inside and everything works again. all fuses were good, replaced the relay switch and worked for while buts now out again.
any ideas of what the problem may be and any advice to how to fix it would be greatly appreicated.
thanks in advance

[mudfuel06]

It may be in the vacuum lines, I think that year had vacuum lines to actuate the hubs from inside the cab. Check to see if one of the lines came loose or is pinched somewhere.

[06DieforDiesel]

ok thanks for the help. one more question, where can i find the vaccum lines? just the general area of where they are

[mudfuel06]

Don't hold me to this, but you should be able to trace the lines from the front axle back.