My 2002 F-250 diesel super duty is tops but it is getting old, 162K, and I am wondering what I should do to make it last a longer. I hear a grumbling under my drivers seat and am told that I should check the transfer case fluid level and that should be done everytime I replace my tranny fluid. Well I have never replaced my tranny fluid or checked my transfer case or rearend for that matter and I am wondering what else I should be on top of. Any regular maint. idea's other than the oil/gas filter change I know of?

Also had a glow plug issue in past, replaced the relay last winter but having similar problems again. Any way to tell if the lines/plugs themselves are bad other than removing the valve covers?

Thanks!