I just copied and pasted this from my thread. Basically your truck has a 9.75 rear with a smaller style axle. The 8.8 bearings work fine since the axle is the same as the 8.8 axle but with 34 tooth spline. Hope it helps
OK, I think I solved it. Axle cost me $240 from ford cuz they were the only ones that had an answer. All of the other parts stores offer 1 axle for the 9.75 and for the 8.8.
The ford tech used my VIN and told me that the tag on the rear end (had i not tossed it) indicated 3.73 gear and 9.75 locking. Also, it said 923A which I guess is the code that says that I have one of the few trucks with a different axle and bearing setup. My trucks takes (driver side) a F75Z4234MB axle. This is identical to the 8.8 axle except it has a 34 tooth spline instead of 31. 99% of F150's with the 9.75 use a F75Z4234LB.
So, I hope this helps anyone with this problem in the future. If you have a 9.75" rear end and the replacment bearings are way too big and for some odd reason the 8.8 bearings fit.......you have the above axle. You didnt get the wrong parts, nobody swapped the motor or rear end when you weren't looking and my personal favorite.....Ford didn't throw in some old parts that they had laying around. For one reason or another ford used a smaller diameter axle on some 9.75 rears from 97-mid99. They switch to the other axle in mid 99.