I have a 1999 dodge 1500 5.2 ext cab..problem is my 4x4 wont engage, its a shift on the fly, any thoughts of what it could be? there is a actuator? on the front axle that I think may be bad..solutions?  

I'm not sure exactly how those dodges engage (I'm sure Sno does and will respond soon) but have you checked the fuses? That is a commonly over looked problem when it comes to this stuff...  

They use a disconnect on front axle (passenger side of diff) that should be vacuum operated. Usually when they fail it is because of a cracked, rotten or broken vacuum line. You might check lines first and then check for vacuum on them when 4x4 is selected.  

I have a 95 ram that had the same broblem, it was due to bad vacum lines, and a bad vacum collector on the transfer case. There is a kit made that gets rid of that stupid vacum accuated ****.  http://www.4x4posi-lok.com thats the posi-lok web site it completely gets rid of the vacum and use a cable to actuate it. Hope everything works out for you.