As far as I know, the stock diff's should be fine for a 10.5 if set correctly, I guess for peace of mind or anything faster the diff's need more frequent rebuilding.
Another option would be the serpent gear diff if it fits, will probably need to find a different length belt though and if its a different size to the your front diff or spool it's not going to be a good transmission setup as you'll get an overdrive senario or something
You might even have a problem with the outdrive bearings being the wrong size too, it could be a lot of work for little gain!