hello, i have a 2004 f150 4x4 super crew lariat with a 5.4 liter. I recently had the rear wheel bearings, axle seals and bearings replaced. The axles need replaced as well from being worn however the guy who replaced the bearings for me says that the rear end looks to have been replaced at some point but he says it was a 9.75 rear end. The autoparts stores around here all say the only model f150 in 04 that come with that size rear end is the heritage edition, im just wondering if in fact i have a 9.75 rear end or the 8.8. it looks larger than my old truck which had a 8.8 rear end im just wondering if their is any way I can check and make 100% sure before i order axles for a 9.75 rear end.

secondly between 48-55mph i get a howling/whining noise out of the rear end it seems, i only hear it when i touch the gas pedal and as soon as i let off the noise stops all together and then as son as i get out of those speeds the noise stops????

 

I have an 04 lariat and have the bigger 9.75 axle.

 

Mine is a 9.75 SuperCab XLT 04 new body style
https://www.f150online.com/forums/ot...-75-8-8-a.html
Check this out for more info

 

If you have the 5.4 you likely have a 9.75 rear-end.
There should be tags on your diff cover

 

I have an 04 with a 4.6L. It is my understanding that only the 4.6L engine comes with an 8.8. Otherwise you must have a 9.75. And the noise you are hearing is ring and pinion noise. It can be caused by an improper backlash setting or just a worn gear set. Either way, once it's making noise the only real fix is replacing the ring and pinion.