OldMansTruck

So my 4wd has stopped working a few times and a little dielectric grease on the plug outside the transfer case solved the problem. It happened again and once again a little grease and it worked again this time it stopped working and grease didn't help, checked all the fuses and nothing, tried shifting into 4wd while moving and still nothing. Well i get desperate and get under there and tap everything i can with a hammer and get back in the truck and now it's shifting into 4wd just fine. Is there any kind of fluid i can put into the transfer case to try to help free up anything that may be stuck or do i have to just drain the whole unit and put new fluid in?