I've been noticing for about good while that there has been a knocking sound coming from under the truck. I can only hear it in low gears or while idling. It also happens in reverse. I got a new transmission about 10k miles ago and that didn't fix it so I'm thinking something is wrong with the diff? I thought about broken u-joints but the look fine.

It's a 2001 4.6L Supercrew 2wd with the 4R70W, I honestly have no idea what rear gears I have, but I know for a fact it is not Limited-Slip.

 

How old are your U-joints on the drive shaft?

 

They are factory, never been replaced.

 

Broken U-joints are easy to spot, usually pieces of them are found lying on the ground near the end of the driveshaft and the cups are sometimes still clamped in place unless they broke while you were mopving ...
... then they scatter.

You can't just "look" at them, you gotta set the parking brake on level ground, maybe block a a wheel, put the transmission in neutral with engine off and the keys in your pocket, maybe lock the doors even, ...
... and then you gotta lay down on your back and go under there and grab that drive shaft with both hands and twist both ways and give it a pull back and forth, at each joint. Any play is way too much.

Or you could put her up on a lift?

Do you have a single long rear drive shaft or is a two piece affair with a carrier bearing? If a two piece, the rear most half is gonna have a slip joint that usually has a grease fitting near by to lube it with.

You should also maybe check torque on the U bolt nuts and it's possible you have a problem inside the rear differential.

 

Oh, tbear.. That sounds awfully expensive lol.

It's a single drive shaft.

I'll check the U-joints out in depth soon, I'm going back to my parents' for the holidays and I can use their garage. How much would a diff run?

I've actually considered putting the 3.55 LS in, would that be a costly swap? Or could I just pay someone with a bit more know-how to do it at a reputable shop?

Thanks for your time man.

 

well you could always hit a junk yard see if they have a full rear axle, i know its not what you need and will take more to switch them out but it might be cheaper then having someone redo just the diff... if i was you i would get a price for both...just a diff swap and the whole axle.. if you have never messed with diff's before they can be tricky sometimes. just my opition but swaping axles to me seems easier then swaping diffs, you do and will need to disconnect everything on your old axle remove it, also bleed the brakes. if you have a lift it will make either alot easier. if you swap diff's it could be very timely if something is rubbing u will have to get all the metal out or it could happen again to the new parts you put in. depending on what the problem is, and if its in the diff, would effect the amount it would cost, could be something simple or major.

edit:
i had a 01 f150 before i wanted a shop to switch to a 3.73-4.10 ls only the gears, i wanna say they quoted me around $800-1000 and that was only for shop charges, new oil, and the gears. this was a few years ago to. good luck