I have a 68 swb that have a truck 12 bolt with 3.73 gears. I want to start hunting for a posi but I'm not completely sure what to look for. Is there a difference between car and truck posi's??? I'm not messing with the pinion. I'm hoping I can just swap the ring gear, reset the pattern and go. What exactly should I be looking for??? Thanks!!

The truck and car 12 bolt are different. The ring gear flange is offset different between them. It is easy to tell them apart. The truck posi has a large window in both sides and the car posi only has a window in one side. You will need a 4 series truck posi to work with the 3.73 gears that you have. You can get a new Eaton truck posi. The new ones only have a window on one side, and are much stronger than the old ones.
http://thumb7.webshots.net/s/thumb3/1/4/61/55410461hkgqKB_th.jpg (http://rides.webshots.com/photo/1055410461041712107hkgqKB)
This is the 12 bolt car posi.
http://thumb7.webshots.net/s/thumb1/1/12/79/55411279rBNihy_th.jpg (http://rides.webshots.com/photo/1055411279041712107rBNihy)
This is the truck posi case. You can see that it has the large window in both sides. Notice where the arrow is pointing. All of the truck cases that I have ever seen had at least one crack here.