I tryed putting my truck into 4hi and when I was driving over 55mpg my truck was shaking badly. Before this I let my dad drive my truck for a while a couple weeks ago and he hit the front passenger side on something pretty much right where the four wheel drive actuator is and I am wondering if it could be damaged from the impact of a crash? My truck does not like going into 4hi or low but if I get it to click in I hear what sounds like a terrible noise comming from the front differential and then the front wheels seem to be enguaged. The weird thing is that both the hi and low light stay lit when in the past if it were in 4hi only the green light would be on. All of my vacuum lines are good and I replaced the bad ones, I am thinking it's probably the actuator or possibly the transfer case switch. Anyone have any thoughts or input?

 

After reading smittys post I'm going to pull the vaccum line off the actuator and feel for vacuum and if I get it then I will check the travel of the cable. Maybe my actuator is messed up and its pulling the cable right between hi and low hence both lights are on. It is stuck in 4 wheel drive, I made sure.
Can someone please tell me what determins hi and low? is it the actuator or the switch?

 

Well after looking at it again it seems like I can get it into low and I hear it clicking in but it does nothing for 4HI. I guess I have all the parts so I will just throw in them in and hope it's the transfer case switch or the 4WD actuator. I know a few other people having similar problems and they replaced pretty much everything I am going to and it still didn't fix it.

Anyone know what the vacuum pressure should be on the line going to the actuator? I'm thinking maybe if throwing parts at it doesn't work I might be overlooking a vacuum leak somewhere.

 

I solved it and hopefully what I found will help someone else because after searching for a while it seems alot of people have problems getting their truck into 4WD HI. I turns out I had a vacuum hose disconnected, when I re-ran a line to the cruise control I disconnected a line from the windshield washer fluid holder and that was the problem. It's crazy that that line being disconnected allowed me to get into 4LO but I could not get into 4HI. So if you are having problems getting into 4HI don't waste your money on throwing in a transfer case switch or a 4WD actuator, instead look for vacuum leaks. Disconnected lines or old lines that are leaking because chances are that is the problem. You can fallow the lines behind the plenum down to the tranny ect. Just look and if you have to you could try pressurizing the system if possible but after looking several times I finally found what I was missing and it was the stupid line comming off the washer fluid holder?