there will be a tag on one of the diff bolts that will tell you if you have a ls or not. There will be an L between some #s...I am sure STL will chime in with a chart, i think there is also a way to check looking at the door jamb sticker...but yes Ford has put open diffs in most of their trucks...I have 2 open diffs. If you do not have an l/s (posi) you can get one and put it in or do an axle swap.
Sold:2004 XLT 4x2 Ext. Cab. 3.0 Mods: 3" spindle, AAL, shackle, 32x11.5 on 15x8s, billet grille, clear corners, flow 40 dumped, line-x, CB/PA, traction bars, lil sound system, auto up down windows
Sold:2005 XLT 4x4 Ext. Cab. 4.0 Mods: ProComp lights w/ HID swap on tabs, 32x11.5 on 15x8s, CB/PA, black out heads/corners, lil sound system, cranked t-bars, flow 40 dumped, line-x, shell, bed rug, 06+ tails, metal tail gate handle since plastic one shattered, OHC, Mustang Dome.