So what you are saying is you want to replace your current differential housing with one that is older and has more miles on it? I assure you that if the '89 had a limited slip or gov-lok carrier, its now junk and needs to be rebuilt. Regardless of the abuse, the functionality is gone after 100k miles. If the old housing has been sitting outside for a few years, the fluid has probably solidified not to mention the seals and bearings that are probably worn. If all you want is a locking differential, it would be much easier to drop in a lock-right. All you have to do is replace the contents of your existing carrier with the powertrax unit. Go to the websites I mentioned and take a look at the instructions!
If you are dead set on swapping out the housing, have the old one rebuilt. It will probably cost around $500 but the time you save will be worth it.