I was told at my recent oil change the differential fluid should be replaced. I agreed as it looked very dirty. I never realized in the past they should be changing the rear and front together since it is always 4WD. When I mentioned this to the employee, he said we should change the front then too. I agreed, however, the fill plug was rusted and stripped and needs to be replaced. However, I can't seem to find a replacement plug anywhere. I have tried CarQuest and O'Riley.
Also, if I drive with the windows up, radio off and AC/Heat off, so that the car is as quite as possible, I hear a very faint sound, which sounds like metal on metal. I now realize the front differential fluid has probably never been changed (now my truck has 97k miles). Could this be the faint sound I am hearing?
Any information about replacing the plug or the sound would be great!!
Typically, the fill plug on the differentials is just a pipe thread plug. If you have it out, take it to your local hardware store and find something that matches. Use Teflon tape or Teflon pipe thread sealer when you install it. Otherwise, just get one from the dealer.
If you're talking about the housing being stripped, then that's a bigger problem.
I wouldn't expect a metal-on-metal sound unless the differential is really low on fluid. I'm pretty certain that the fluid in my '98 wasn't changed prior to me buying it. It had 157k on the clock when I bought it. The front differential doesn't take as much load as the rear.