if your going full width axles then just grab a 9"...
i hear the argument about how low the pinion is on 9"s.... well i put my old 8.8 beside my new 9" and your only losing about 1/2" of clearance... the 8.8 and 9" are VERY close in size and pinion position....
when it comes to welding diff's well... alot of folk's say there great at doing it but few ever really make the grade.... do you trust someone to weld up a carrier your about to bolt $200 worth of gears to? knowing if it blow's yer out that $$$$ plus the install $$$$ and time without yer rig and even having to tow it off the trail? spool's lookin like cheap insurance now eh?..... on the other hand theres always someone who's ran 9's in the 1/4 or 44"s in the bog's with a welded diff and never had any issues.... i think welding a diff is a crap shoot.... have i done it? yes i have welded a few carriers and there holding up fine.... but they are D60 carriers and have alot of meat in them and all the trucks have 35"s or smaller on em...
i run a spool in my rig.... yes it chirp's tires, yes it puts alot of strain on the driveline, yes my truck feel's gutless on pavement due to the axle's always putting drag on the driveline.... BUT... my tires always turn, i dont have to wait for 1 wheel to slip, it's stronger then any regular locker, it's atleast twice as strong as a regular carrier... in short i LOVE it offroad.
8.8"s are a pretty stout diff but they cost a certain ammount to make em strong.... 9"s are dert cheap to build and are proven to be one of the strongest diff's on the planet.... there is more tooth contact with a 9" then a D60 and as ratio's get lower other pinions get smaller when the 9" pinions only get bigger.... you wont break a 9" with 31 spline aftermarket axles and a full spool that has been setup by someone who know's what they are doing with the 4.0 and 36"s.... i run 35" claws at 25psi with a stock 81 F150 9" in the back, full spool and 4.56's mostly trail but i drive it there and back on the street, so far no issues to report....
and if you look thru the worst stuck thread you will see i drive er HARD.