Clean off and warm the spiders up some with a torch before you weld the snots out of them. Don't be stingy with the bead and make sure you get good penetration into the gears.
Had a 2wd F250 we killed off in the woods on ATV trails on my neighbors land, It was worthless until we welded up the spiders. You still had to use a lot of momentum to make it through the mud, rocks and trees but it did pretty good. The long posi breakstands we did for 1/4 mile down the road looked great too.
Oh yeah, cheap SOB trick for a welded diff: drain the gear oil into a zip-lock bag when you pull the cover off. Pack the bag back in around the carrier once things have cooled off and put the cover back on. After a few miles there isn't a trace of the bag left, the ring and pinion make sure of that. Great for the beater truck with a budget of $0.