It depends if your doing gears. If your just doing the rear axle differential then for costs: true trac $500, Installation kit w/bearings shims etc $150, labor to install $250+..so about $900 and you can be all set..
if you got big tires or plan to tow constant heavy loads or do a lot of off roading, it may be wise to grab some 4.10s or 4.30s gears, but you will have to do the front and rear.
My set up:
FRONT: true trac ($590, ebay), installation kit $60, 4.10 motive ring/pinion $110, Amsoil 80/90 lube $30, installation labor $400
REAR: Detroit Locker ($650, ebay), installation kit $169, 4.10 motive ring/pinion, $175, Amsoil Severe Gear 75w/110 $55, installation labor $400
Under $2700 in central texas
'07 F-250 PSD 4x4
5" exhaust, zoodad, blue spring, Airaid intake, 6.4 starter, 2.5 f/coils, adj Trac bar, 2" r/add-a-leafs, egr delete, snow plow fan clutch, csf 6028 intercoolerr, DC 185 alt, Bilstein 5100, Barder '03 turbo rebuild w/RR billet wheel, SCT SF3 tuner, F-Detroit Truetrac, R-Detroit Locker, 4:10 gears, hawk LTS, LT315/70r17 Duratracs, Amsoil Gear lube, Delo 400 LE 5w30 diesel, Delo ELC, archoil, bpd ficm, BPD oil cooler, dynatrac pro steer ball joints, arp studs, oem gaskets, HD water pump, stc, plugs, stand pipes, 6.4 banjo bolts,