Hey new guy here. Truck is a '99 4x4 4.0 w/ 8.8" 4.10s and had it almost a year now and have used the 4x4 high and low before (about 4 months ago). Long story short, 4x4 doesn't engage. I've researched all over this site and others about common problems with the crappy PVH hubs. I took the hubs apart today and cleaned them out, one was filled with what looked like mud and grease and the other was spotless. 4x4 still no good, so the vacuum system is probably shot. Heres my plan, I'm gonna go into the hubs and manually lock them up and re-test the 4x4. If that works I'm going to buy the Rugged Ridge manual locks so I can lock and unlock the hubs without taking apart the stock hub.
Next; while I wait to diagnose the hub issue I have an actual question I couldn't find answered in the forums before. While I'm driving down the road, under 30mph, and I flip the 4x4 **** to 4hi there is a tapping/rattling coming from somewhere under the cab that matches the speed of the truck. When I come to a fully stop there is a slight clunk but still no 4wd. Any ideas about what that could be?