The axle will bolt right in, but the higher capacity springs would be needed as well. Now keep in mind that just because you're changing the rear end does NOT mean you can carry more weight legally. You can only register the truck up to the mfg specs. If you're constantly loading to that level and needing more, then you NEED an F350. What you're trying to do is put a bandaid on the problem.
99 F250 PSD 4x4, 5" DIY exhaust, CCV mod, DIY intake, PMT1 with DP Tuner chip (stock/40/80/120), BTS VB, Suncoast Extreme Trans with SD43 converter, FASS fuel system.
Trail Rig: 95 Ranger on Kingpin D60/14 bolt/D&D Doubler.