I did the removal/installations (3 now). As far as who built the original diff, I would rather not go there. For all I know I just screwed up on all three installs, so I don't want to place blame.
I had a mechanic check the first differential right after the 2nd install because of the whining noise, he showed me that the wear pattern was PERFECTLY centered, could not have been set better. He advised me that the whining noise was definitely a bad bearing and the backlash was correct. The builder of the diff gave me the phone number for the carrier manufacturer and left it to me. Later he offered to tear it down for a flat rate for labor and then he would repair, but it would have cost me $100+ just in shipping and I already had $1100 in it to start.
This second diff that is in the car now was purchased from a TCCOA'er who stated it had 12K miles on it. I had this one checked too by a mechanic before install. Again, backlash and gear pattern dead on and he said that it looked to have low wear/mileage so the 12K miles was plausible.
Immediately after install, like I said in the OP, diff was incredible, smooth, quiet, tight. Since that post it is obvious the problem is slowly getting worse just like with the first diff. The most noticeable difference is in the shifts. When I first installed this TL diff, the 1-2 shift would chirp my tires hard even at low throttle, with no clunking in the rear, like it was tight. Now, it takes moderate (obviously more) throttle before tires chirp, and the shift is immediately followed with a clunking noise in the rear, and I can feel play/shift shock like its just getting "loose". I guess I could imagine this stuff, but I doubt it. I was hoping the play was just control arm bushings or something, but the noice is apparent when turning the rear tires to turn the axle to turn the diff (like I said in OP)
I have accepted the fact that I will have to live with the clunking until I have figured this problem out and decide to replace it. I will probably go with 4.10s next time just because I have to do all this over again, but I hope this diff lasts as driveable for 2 years or so, like the first one.
The point of my post is to try to figure out why this is happening. The only thing that I can think is that the axles were damaged with the bad side gears in the original install of the first differential - but I inspected the axles before installing the second diff, and they look fine. I tried to buy new c-clips but they are no longer available at the Ford dealer here. The originals looked fine anyway, and other TCCOA'ers confirmed they have re-used theirs before too.
P.S. Splattered - Yes, I have done many repairs to this car. There is not a speck of rust anywhere, I just did all the suspension (just need to do upper shock mounts now). Like my sig says, I have replaced the whole drivetrain, all mounts except rear subframe mount bushings. There are a couple of minor things to do, getting a new windshield monday, brake and trans fluid are due for flush soon, and my SC 145mph speedo gauge odometer gear needs to be replaced, but the door hinges are surprisingly still ok. But overall this car has been cared for well.
All Glory to God!
RIP JOHNNY LANGTON
96 LX PI/Jspec Pearl White. She enjoys eating ricers for breakfast
2002 PI 4.6L - Jspec Trans built by Darrin - 04 Marauder converter - Hayden 679 Trans cooler - Engine cooling modification - SCT Xcal2, tuned by Lonnie - 02 GT MAF and 99 GT Intake tube - removed silencer - 255 lph HP fuel pump - Aluminum Driveshaft - 3.73 TL - 245/60/15 BFGoodrich Radial T/A's - PBR front brakes - SCP Rear torsional load brace - SCP Solid rubber motor, transmission mounts - Rear Strut Tower brace and Diff cover brace by Rod
Last edited by 96_LX; 02-06-2010 at 08:30 PM.