'95 GMC Jimmy (S-15) 4.3 L (W) 4 sp auto push button 4x4. Will not go into 4 wheel drive sometimes. When 4HI button is pushed, either nothing happens or 4HI and 4LO buttons flash for a few seconds, but then 2 HI button re-lights and it is still in 2 HI. If I disconnect the battery cables for a few minutes, it will " usually" (not always) work right the first time it is engaged, but after it has been put into 2HI, it will not return to 4HI again. Sometimes, it works perfectly, just like it did when it was new. When the truck is in four wheel drive, everything works perfectly and it will stay in four wheel drive until the 2 HI button is pushed again. I have had no trouble getting it back into 2 HI from 4 HI. By using the override method of pushing 4LO while in neutral and then putting shift into “PARK” while control panel lights are still flashing, the truck will go into 4LO; however the transmission will not shift through all gears when in 4LO. The front axle actuator (the vacuum operated unit under battery tray) has been replaced and is working properly (cable is not sticking, etc). On occasion, the lights on the control panel do not light up. I know this is because of the vacuum switch on the transfer case is faulty. The switch that tells the ABS brake system whether the vehicle is in 2 or 4 wheel drive was replaced by a dealer per GM recall. . It is my understanding that if the encoder motor were faulty; it would not work at all at any time. I have a Chilton repair manual but no information about the electronic/vacuum systems of the 4x4 is available in it only mechanical systems. Is this as simple as a loose connection somewhere? Is there a repair manual that gives info about all components for the 4x4 system my truck has? Thanks for any information anyone can offer!