I have a 2007 4x4 with a 3.73 8.8" rear differential.
The pinion shaft broke and chewed up my pinion gear, as well as that the pinion seal is leaking. Replacement gears, bearings and seals would cost me around $800, maybe a bit more.
I can get a deal on the entire rear end with the same 3.73 gear ratio. But the ring gear is 9.7 inches instead of the 8.8.

My question is, can I replace this without any issues? I guess my main concern is, does the larger housing require a shorter drive shaft or should I be able to just do a straight replacement. Does changing the rear end in this way have any negative impact on the 4 wheel drive? Would there be any difference in axle length?

I'm hoping that as long as the gear ratio is the same, since it's from basically the same truck, that there should be no problems.

I should add that my existing differential is NOT limited slip. And I'm not sure if the used one I'm considering is. Would that be an issue if it is?

 

As long as both axles are the same gear ratio your good that way. Not sure if the drive shafts would be different lengths. Does the "new" axle have the same kind of brakes as your existing axle? A rear drum truck uses a different master cylinder vs a 4 wheel disc brake truck.

Is the "new" axle from the same type and year of truck? There is interchange information available, if you can't find it online you can call a salvage yard and they should be able to tell you if it will bolt in or what has to change. As always (even here) the more specific info you can give the better chance you have of getting the right answer back

 

Wow I never even considered that. That definitely would have caught me, but luckily yes, both old and new axles have disc brakes.

 

Almost all vehicles are using 4 wheel disc now, but not ALL. And they master/ ABS and proportioning valving are different even if only minor it could cause issues.

If your replacement is coming out of a similar F150, a quick measurement (if it's in a truck still) of it's driveshaft and your own will answer that. You could also measure the distance between where your axle mounts to the springs. And take that along to measure the replacements mounting points (from side to side)

 

I am more familiar with my 2003, 1997-2003 style, and while my truck has the 9.75 axle they also came with the 8.8. Not sure if Ford changed much axle wise after that, but if the driveshaft is shorter I can't imagine it would be too hard to locate one that fits at a salvage yard. Or a shop that could shorten your existing one for a reasonable price

 

Ya I thought of taking measurements. This rear end was taken out of a totaled F150 so I dont know if he still has the truck or not. And about an hour and a half drive away so it's quite a way to find out it's different. I'll give him a call and see if it's possible for him to measure the drive shaft. Thanks for you quick replies.

 

Good point. I spent all last Saturday at the local Pick n Pull looking for something with the 3.73 gear ratio with no luck. But at that time I was hoping just to replace gears and bearings, so I was only looking at 8.8 differentials. If I have the option of 9.7" I think they had a wider selection to pick from. Although I think the newest F150 they had was probably a 2001.

 

If you have a 8.8" rear, you have a 4.6. The 9.75" rear was used with the 5.4.

 

glc, did the 4.6 come with taller gears? OP stated a 3.73 ratio. Also, since I'm not sure, were there any major changes to the rear axle that the OP couldn't use an axle as far back as say 1997? just want to add to my knowledge, and help the OP find a good answer

 

I don't think you can use a complete axle assembly from the previous generation truck - and ratios were not engine-dependent.

 

As long as he get a rear axle from the same generation truck they are a direct bolt-in except for the driveshaft. The spring perches are the same, the brakes are the same, and the parking brake is the same. At least on the 04-08's and the 09-14's. Some of the 97-03's had rear drum brakes and some had rear disc. In 04 they all got rear disc and 6x135mm lug pattern, prior to 04 they only had 5 lugs.

Actually, I know for a fact an 04-08 axle will fit, and there is about a 90% chance an 09-14 will fit also. The only part I'm not sure about is the distance between the spring perches, and I think the 09+ had slightly bigger rear brakes than the 04-08's.

 

good information 05REDFX4, glc. I wasn't sure if mounting points got changed, if 04 and later F150's were all 4 wheel disc, or (although I was about 80% sure) driveshaft lengths would be different. I was certain about having to have matching gear ratios front and rear.
That's the beauty of this forum, I learn something almost every time I'm here.

 

The truck you mentioned is a 4x4 and most all 4wd's from 04-08 had either 3.55 or 3.73 gears. 2wd's can have a range of gear ratios from 3.15 up to 4.10

If the axle is still in the truck, look at the sticker on the door jamb for the axle code, a letter then number indicates a limited slip diff and two numbers is an open diff. 3.73 geared trucks use the number 6 as the second digit in the axle code. Also all FX4's from 04-08 have the bigger 9.75" axle and LS as standard equipment.

 

You can also read the tag on the diff, if it's still there. In addition to "88" or "975" you will see "373" or "3L73" if it's a 3.73, "355" or "3L55" if it's a 3.55, etc.

 

here's an example of the tag glc is mentioning

it's a 9.75 3.55 limited slip (in my 2003 F150)