Manual locking diff? what is it?

i got a 93 4.9L f150 5spd. i just found out that i got a axle code 18(3.08 gears and a conventional diff.) i was screwwing around with my friends and got stuck in a snow bank and only the passenger tires were spinning. i had it in 4x4. i was just wondering if it is expensive to put a LSD or manual locking diff in? what is a manual locking differential? i want to look into this cause if i get stuck in some snow drifts i am screwed. and also in the back drivers tire i heard a griding noise from the tire.

one thing is in the summer i would go around the corners and give it some gas and it would jump around? my friends truck did the same thing and he has LSD Diff.

A manual locking diff. to me means you need to get out of the truck and turn the manual locking hubs to the lock position.(on the front wheels).
Make sure the locking hubs are both in the lock position (or free position when not using 4X4 mode.)
It is not uncommon for the hubs not to engage properly. Cold weather, too much grease or not enough grease can cause this.
If you continue to have problems, remove the manual locking hubs for inspection to make sure both of them move freely.