i believe the jb weld is normally used to lock a centre diff. locking the funfactor diffs will more than likely have an adverse affect on the handling, depending on what u use it for... this would apply more to the front than the rear.

i packed mine with grease after filling the diff casing holes. i have a kinda limited slip effect going on at each end now. if u are trying to stop the inside front wheel spinning, then try making up a rear anti roll bar, all the mounting points are already there... it worked well for me on my rally.