Melt down with 500 lbs!!!!!!!!!!!!!!!!!!! Now that is really good for a laugh. I have just completed a repair on my high miles 93 rear axle. Only got 500k miles. Actually this is the second time in about a year (and in the 500k miles).
I was pressed for time and did a patch job about 40k miles back. I didn't pull the pinion and I should have so this time I did it all. You don't say what your miles total is but unless you have experienced a low oil problem or racked up LOTS of miles, you would have to be very unlucky to be about to have a problem. Things do happen though. I have an axle with only 180k miles that had the bolts that hold the (LSD) diff halves together start to back out. Only four were tight. Three were backed out and bent. One was broken out and pieces in the bottom of the housing.
So do more than look at a dipstick. Remove the differential cover on a minimum 100k mile interval and look it over good. Check for side play on the carrier. The bearing on the gear side is the weak link. I have another axle that almost lost the gears because of the gear side bearing going out. In fact a 50k mile interval would be better, more as the truck racks up high miles. In the pre diesel days, the dana 60 or 70 was quite good for the full life of the engine. But with the torque monster Cummins (I stole that phrase from Michael Miller) the 70 is going to need some service before the engine dies. IE, this is not your daddy's truck.
I mentioned 'weak link'. Actually the exceptionally weak link is/are the rear hub seals. As Tugboat can verify. And that leads to the next comment.
I check my transmission and differential every time I get out of the truck. How you ask? It is really quite simple. transmission and diff lube has a very distinct odor. Tune your senses to anything that even smells out of the ordinary and you will know right away that there is a problem. When you get back in the truck, ALWAYS notice under the truck. There should be some water from the AC and there should be nothing else in the way of a puddle under your truck. If the wheel seals are leaking, there will be odor and there will be wet (actually oily) patches on the insides of the tires. I do this even on the low miles 93. It is just second nature to me as I have always been the one who had to do the repair as well as do the paying for whatever parts were required.
Keep a healthy mixture of antifreeze even if you never experience freezing temps. It will alert you in the same way if there is a coolant leak.
Been a while since I got any warm fuzzy feeling looking at my trucks but I see one with fresh paint now and again. It always gets my attention. I am putting some miles on a 97 but I am definitely a first gen man.
James
Attention: TDR Forum Junkies 
I only haul about 500# a few days a week. Thanks for any insight 
TDR is a great resources, and for a good laugh. "Stop turning the light on in driveway, and going outside every fifteen minutes, you can play with your truck in the morning. " :-laf 
. My truck has 210K plus miles right now 
