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With an unknown history, that would be a good place to start. I have had this help other auto trans vehicles I have owned...
I have a 2004 with the same engine, but fewer miles. I have read in many sources that the designed-in life expectancy for the engine was 150,000 miles.....
The 4.7 in the Ram (at least for 2004 model year like I have also) calls for copper spark plugs only, that need to be replaced every 30,000 miles. It's easy and I did it myself in about 90 minutes. I have read mutiple threads that warn NOT to use anything else, as does the owners manual, no platinum, etc. I don't know why, my aunt's 2004 Jeep with the same engine DOES use platinum plugs. All eight copper plugs were $12 at the dealer. I only have 31,000 miles on my 2004 but every time it moves it is hauling something or pulling an enclosed trailer or a landscape trailer for a property company. I just changed both diffs, the transfer case, transmission, and coolant out for the first time, going by age over mileage. I would think these should be done every 40,000-60,000 miles, though the manual says the coolant can go 60 months or 100,000 miles I think. I change oil and filter every 5,000 miles or about once a year, and use Mobil 1 5w30.
My 2004 Ram 1500 just started having no indicator light either, but the 4WD works fine. The owners manual says nothing about lack of a light being a warning of any sort. There is a separate "SERVICE 4WD" light that warns of a problem. So, I think I am going to ignore the lack of a light.....
I have a 2000 S500. There are four cabin filters total, two fresh air, and two charcoal filters. They are accessed be removing the trim panel under the passenger side of the dash, the plastic panel up under the dash past the carpet trim under the glove compartment door. The charcoal filters are behind one door, and the fresh air filters behind another. You have a W220 chassis. If you google "w220 cabin filter replacement" or something similar you will find lots of good instructions from do-it-yourselfers.