You probably want a Limited Slip(L/S) differential vs. a Locker
Limited slip(GM calls it Posi-traction) allows each rear wheel to turn at a different speed, like when you go around a corner, but both wheels get power all the time.
Open(regular) differential is when only one wheel gets the power.
A Locker does what it says, it locks both axles together so both must turn at the same speed, making cornering on a hard surface difficult, one wheel will start scuffing, or even worse, something will break.
On gravel or dirt it is OK, so really only for off road use.
On your drivers door there is a Label that will tell you what you have now.
Like "axle 86"
Which would be an open 7.5" with 3.73 ratio differential
Look here to look up what you have: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
Since you have 2 wheel drive you can also change the ratio if you want when you change the differential.
You can swap the whole rear end or just the guts of the differential, your choice.
If you really want a Locker then google: POWERTRAX LOCK-RIGHT
This can be install on open differential and is an "automatic locker" so similar to Limited Slip/posi-traction