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
. S&B, 5" exhaust, zoodad, blue spring, '03 Turbo, 6.4 starter, 2.5 f/coils, adj Trac bar, 2" r/add-a-leafs, egr delete, DC Power 185, Procomp shocks, SCT SF3 tuner, F-Detroit Truetrac, R-Detroit Locker, 4:10 gears, LT315/70r17 Duratracs, Amsoil Gear lube, Amsoil ss fuel efficient ATF, Delo 400 LE 5w30 diesel, archoil 9100/ar6200, bpd ficm, arp studs, oem gaskets, HD water pump, stc, plugs, stand pipes, 6.4 banjo bolts, monitor, new oil cooler, coolant filter