The truck engages intermittently in drive. The 4wd light stays on but it feels like the actuator doesn't fully engage because at times I can feel a pop in the front end, and I no longer have 4wd. I have replaced the actuator and checked that the collar slides back and forth between the axels with no restrictions. The axel splines look to be in good condition. I have vacuum to both lines for engage and disengage. The indicator light switch is mounted on the axel at the end of the actuator. The strange thing that I cannot find an answer to is that it always engages fully in reverse. The vacuum pump has been rebuilt but a year before any problems surfaced. I did find a month ago that one of the vacuum lines had been knocked loose. I hooked it back up, but it did not seem to change anything which seems weird in its own right. I haven't checked the vacuum amount, but I can see how there would be more vacuum in reverse than drive. The vacuum at the actuator seems to be the same as it is at the vacuum pump. No noticeable drop. Someone has to have encountered this problem before.
Try putting in neutral before shifting to 4wd it may have preload torque on it from driving on dry pavement to long. It may be an adjustment of shift linkage and or binding or a n internal problem. I tried to copy shift adjustment but can't do it so you'll need a book or manual to do it.