'92 Dakota front diff. lock

The last few times that I've engaged my 4wd on my '92 Dakota the axle refuses to disengage. I try all the tricks -- back up, power on, power off, etc. and eventually I have to get under the truck and remove the four bolts holding the disconnect housing cover, pull it out a little and manually sliding the shift fork & collar over -- disengaging the axle. Needless to say, I'd like to find a little easier way to disengage it. Is there a history of a particular trouble spot that I should look at first? I thought that I would start at the vacuum can. that is supposed to pull the collar over automatically. In my book it is called the "shift motor assembly". Just to clarify one thing -- my problems is not with the disengagement of the transfer case but rather, the front drive axle. Help