1. Sorry.
2. Cars are not an investment. Unless you intend to keep a car for 50 years in prestige condition, or you think maybe, one day it will be a rare keeper. 99.9 percent of cars are just that, cars. They are not stock options.
Because of this, you gotta get your head away from thinking things like that. If you do, you will always be upset. Hell, buying a new car would make you sick since the moment you sign the paper and drive it off the lot it losses about 20% of its value. BAM!
3. You can't determine worth like you did. While it would be NICE to say, "LOOK! I put 3k into it, that makes it worth 3k more, right?" It doesn't work that way. In particular, the Transfer case. When you simply repair a broken part, the value doesn't go up from what it is market value is considered.
Occasionally you may find someone who values some aftermarket goodies enough to pay extra for it, but in general, it doesn't work that way. New tires don't increase value. New spark plugs don't increase value. When people look at a car to buy it, the SUBTRACT for ****ty care but they don't usually INCREASE for proper care. A well maintained vehicle is worth more than a non-maintained, but not above what is usually considered market.
Sure market is all about what people will pay. But, you bought it for 11k and so I have to assume that is close to market. Maintenance isn't going to increase its worth. It def. isn't worth 13-15k. I bought a newer and in better condition truck a year ago for less than that.
All this is to say, if 11k is considered fair market in your area; you won't get people willing to pay above that because you replaced the transfer. In fact, they may actually ask you to lower it since the truck NOW has proven history of mechanical issues.
This is why on older cars when the total value of the truck is 3 or 4 k, it is sometimes not worth it to spend multi-grand on a new engine if you are thinking resell. It isn't worth it.
Finally. If you have developed a hatred.... and some serious buyers remorse, then do sell it; but be very, very, very realistic in what it is worth.
As for underwater. Did you finance? On a truck that old, that is unwise; simply because it sucks to owe 11k on an older vehicle that is starting to have issues. But if you DID... I suggest you pay as much as you can to get the loan value below the fair market.
If you own it outright, then your not underwater. Keep it till it dies. Also, anytime you buy used, you will discover stuff about it that irks you. My seat creaks and my factory rims are starting to bubble...
But, like others have said, if you it runs (and now you know your transfer is good)... then keep it.
You won't get ANYWHERE near 15 for it, so you will take a huge loss.