What should the back lash on a 2004 F150 rear diff 3L55 8.8 be? On the spider gears

suggest that you talk with a diff reconditioning specialist center
if you have excessive pinion to ring gear clearance the diff will be making terrible noises not a clunk
what you have is worn side gear washers in the diff carrier and that is not essential to repair unless you get diff noise on turning corners
All diffs have play in the diffs that most people say is pinion /ring gear problems when it is not
the clunk noise comes from engaging a gear and the diff housing reacts to the load ( radius rod bushes , stabilizer bar bushes etc), the side gears take up the clearance, and the
"U " joints or cv joints take any wear
Best advice is--- if all you have is a clunk ( check everything outside the diff first) and the diff runs quiet under acceleration or deceleration, DON'T fix what ain't broken

the spider gears ( side gears ) are inside the differential carrier and are easy to assemble. If you mean that you cannot get the diff centre back into the housing, then you will have to use the special housing spreader to push the sides apart enough to get the carrier back in.

Most likely you have chewed up spider gears , the 4 small gears in the carrier. Remove diff cover , inspect gears for pitting or grooved slits.

This is a hard job to do,l always fit the crown wheel as well.99.9% of the time if the pinion is shot so to is the crown wheel.When refitting these units it is very important that the back lash and pre-load on the diff.are set to workshop manual tolerances or the diff.will fail again..Good-luck...

First thing is to make sure you chock the front tires so the truck will not roll. Raise the rear end and set it on jack stands. Put the transmission in Neutral and make sure the parking brake is off. Remove the rear tires and disassemble the brakes on the side with the leaking seal. Place a drain pan under the rear differential and remove the cover. Remove the differential shaft lock bolt. You will need to turn the drive shaft by hand to access the bolt. It will be on the right side off the spider gear housing. Only use a 5/16 or 8mm 6 point box end wrench to remove the bolt. After you remove the bolt reach on the top of the spider gear housing and push the differential lock shaft down to remove it. Make sure the spider gears do not rotate after you remove the shaft. If they do you will need to realign everything to reinstall the lock shaft. Push the axle shaft in enough to remove the "C" clip from the tip of it. The axle will now slide out of the rear end. You can use the tip of the axle shaft to remove the axle seal. If you're changing the bearing as well you will need a slide hammer with axle bearing adapters to remove it. You can use a socket and hammer to install the new bearing and seal. Be careful not to damage the bearing and seal. I recommend packing grease in the back side of the seal. This helps prevent the spring on the seal lip from coming off. Assembly is the reverse. Clean off all the gear oil from the differential cover and the axle. You can use RTV as a gasket if the OEM gasket is damaged. Fill the rear end with new gear oil of the proper type. Refer to your owner's manual.

sounds like your spider gears are bad in your rear diff. common.. chrysler 8 1/4 diff..

Get the rear wheels off of the ground, remove wheels, and drums. (Rotors if you have those)
Remove rear cover (use a drain pan to catch fluid)
Look for the 5/16" or 8mm head pin bolt and remove it.
Turn the differential so you can get the pin out. (goes through the center)
Push you axles inward and with a small magnet, take both "C" clips out from the inner part of the axles.
Now slide out you axles. (Careful on the outer seals)
Mark your carrier bearing caps so you know what is left or right, and remove them both.
Find out what size the bolt heads are for the "ring" gear and get two wrenches to fit. (Usually 19mm or 3/4")
Hold one on a bolt head on the top side and put the other wrench lower.Force the lower wrench upward, as this will push the upper wrench into the housing and "push" the differential out. When removing be aware that there are spacers that need to go back where they came from. DO NOT MIX THESE UP. These spacers are under the carrier bearing caps, left and right of the differential.
Note, four spider gears is a normal differential, requires gear oil. If it has clutch packs, it is a limited slip and requires gear oil and limited slip additive!!!

remove wheels, brakes, diff. cover, and slide out spider gear pin. the pin holding the little gears in place in the diff.. this probably has a screw or bolt holding it in. remove spider gears upper and lower with bushing plates. push axles inward to remove the big c clips, slide axle out. be careful not to damage the axle seals with the teeth on the axle. reverse order to reinstall

You must pull the diff cover off, then remove the center pin and spider gears, then remove the C clip that holds the axle in the carrier assembly.
Please rate me.

do you know if you have a limited slip rear diff?
if you do your grease is worn out or if the fluid was replaced it need to have a friction modifier put in the diff (can be found at any auto part stores.)

if its not a limited slip diff you might have spider gears going bad. then you need to remove the cover on the diff and inspect your spider gears and these are the spider gears
and this is what a limited slip looks like
good luck
and dont forget to rate my answer
thanks
paul