I know that it all depends on the driving. I know chevy 10 bolts in truck seem to break all the time same with the ford 8.8 but under normal driving and towing condintion no slaming gears and what now how oftern to you hear about d60 rears going out or needing rebuilt. Sam with the d44 closed knuckle front. That truck that I am buying to morrow has been sitting for a few years. The guy go it running pretty good but has only drove it around the parking lot has never took it on the highway. I just didnt want to get going down the road and the rears blow out in it. The guy who owned it first used it to tow alot but it just seems like these old fords are tough. By the way its a 73 250 4x4.

 

Can't say I have a ton of experience with them but i have abused the hell out of mine.. in every way possible.

I bought it, did a lot of offroading when I was an idiot (hardcore stuff that most smart people wouldn't do to a vehicle they intended on keeping), then got bored of it and let it sit. 4 years later I started it up, fresh tank of gas and it started fine.. first thing I did before I checked anything, I went out offroading. Drove it through a lake, went up and down some hills, some really hard dried up mud holes, anything you can think of. Drove it around for a few more weeks before I learned my (or any) rear diff needed oil, so I added oil (it was empty).

Since I bought the truck it had a 'clunk' whenever I would shift gears, I just assumed it was a weak transmission.. when I added gear oil that 'clunk' went away, so maybe I had been running with no gear oil for those 8 years+? It wasn't a lot of miles, but it was a lot of very hard miles.
Anyways, I have a rear pinion seal leak but otherwise my rear end is fine. In my limited experience, the Ford 9'' can handle a lot of abuse.

Oh, forgot to mention that this whole time I was driving with two broken U-Bolts on the driver's side.. it's a miracle the axle wasn't ripped out from under me, not only that my rear axle was off-center and angled inwards because of this.. because I drove for so long with it like this, I'll have to replace my leaves.. but my rear end is fine, no play whatsoever, no noises, smooth as can be.

knock on wood, now that I said all of this my rear end will probably go out tomorrow.

 

My 1977 F150 had the 9" Ford rear in it. The truck had 320K miles on it and the rear was still quiet. That truck spent 1/2 of its life overloaded and the rear never quit. Maybe it lasted so long because the truck was a 6 cylinder?

 

As long as you keep oil in them, and change it regularly, it should never wear out. My dad is the king of vehicle negligence, and he has burned up two 9" rears because he never checks the oil, and has never changed the oil in one in all the time he's been driving them. The newest truck he's ever owned is a 1971 F100 4x4 and he drives it every day, its completely restored and he has restored several but he never takes care of them past that.

 

With proper maintenance, keep it clean, eat lots of green vegetables and avoid spicy foods and have a colonoscopy after the first 50 years you should be able to get a life time of good service out of your rear end.

 

I bought an old GMC 1 ton about 15 yrs ago that had been used on wheat harvest pulling the bunk house and grain wagons for years then used on the rodeo circiut pulling a horse trailer on a pro circuit then after that used to pull a trencher for another 4 yrs, was on it's third engine, second trans (old 4 spd) Due to the mileage the wheat harvest guys put on it every year it had rackedup quit a few, as it turned out the base chassis truck had over 500,000 miles in the first 10 yrs of it's life, couldn't quit nail down how many in the 5 years prior to me buying it on top of that but it had a D70HD in it, and when I bought it I decided it was too high geared and it was an open diff so I put a detroit and lower gears into it. first thing we are 99% sure the oil have never been changed in it, the side bearings were just starting to show some wear patterns slightly the gear pattern was perfect, the back lash was still set at .016 with is a little towards the high end for that diff but not bad considering and was still quiet.
Now this truck was loaded and pulling heavy 95% of it's life, had even had the rear spring pack replaced at some point and I replaced them again while I owned it. S
So does that give you an idea what a diff can take and how long?

 

the average joe driving his truck, under normal conditions, and doing regular service maintanance on the vehicle the diff should last the life of the vehicle no problem. that being said, we are not all the same.normal for one person is abnormal for the other guy .my truck had never failed the original owner.then i bought it,and broke the diff in the first year i owned it! i have put a much more powerful engine in it. in doing that,and running the truck to the new engines capabilities,the diff and trans failed.(a chain is as strong as its weakest link).now ive upgraded the whole drivetrain to work well together. a stock trucks drivetrain is matched to work well together."the whole is the some of all its parts". if your hard on your truck,it will at some point say no more abuse. for the most part these trucks are very very strong, and will be forgiving to us for running them hard.everything has a breaking point, cross that point,it will break. you will be surprised on how tough these trucks are stock.just dont expect to beat on them and not have wear and tear issues at some point.-billy