Ok so full disclosure on what's going on,open 8.8 rear in a 93 f150 auto 4.9. It ran low on gear oil and the drivers carrier bearing failed, not too bad though. I've replaced the axle seals/bearings and have not touched the pinion... I tore out a junkyard matching 3.08 differential in great shape,I've left on the junkyard abs tone ring/3.08 gear and I took the drivers axle and race/shims also...

My issues, firstly my driveshaft is not spinning by hand/screwdriver(although not really forcing it cause I don't wanna hurt my ujoints) this really bothered me!! so I put a bunch of gear oil on the pinion and threw it in drive for 5 seconds, my driveshaft spins...but again not by hand,(in neutral, on jackstands) don't know what's really going on here except possible bad bearing in the pinion but I really don't think so, is it visible with a mirror or something? I really don't feel like taking off my driveshaft.

That issue is what is obviously causing my next issue, I got the carrier in with the old bearings still pressed on(going limited slip very soon, I need it driving first though) I used the junkyard race and shims on the same sides. The carrier is very tight, I punched the last shim in with my old shim. I got the axles in and the spider gears spin.(yes!) Can't spin the driveshaft or gear though(damnit!!!) I don't have a dial indicator yet... what are the chances she drives as is??? I can't check the tooth pattern but the axles are in and there is ZERO play on the carrier, left right up down and I don't even have the race caps torqued down yet. Stumped, any help would be appreciated.. really not trying to mess with the pinion here, or really measure the backlash, if it's a little noisy I can live with it, if it makes it to the trans shop 10 miles away I'll be happy... thanks in advance

 

I can't really advise you but I can tell you my experience of many years back. I had a 1988 F150 with 3.08 gears and a 5 speed. I could hit 35mph in first gear, but it was worthless trying to pull a trailer. I ended up swapping the gears in it but had absolutely no idea what I was doing, and had no clue about checking tolerances, etc. I threw in the 3.55 gears, bolted it up and off I went. I drove that truck pulling trailers for the next 3 years with no issues whatsoever. Then one day I made a U turn with a bit of throttle and the entire rear end exploded. Pieces of gears came out through the rear cover. I ended up buying a junkyard rear axle with 3.08 gears and swapped it.

That was my first and only experience trying to change gearing by myself. Ever since I've paid a qualified mechanic to do it for me.