I'm gonna try not to sound really uninformed as I describe this problem (I'm not mechanically inclined) but I need help making the decision whether to put a new transmission ...

I'm gonna try not to sound really uninformed as I describe this problem (I'm not mechanically inclined) but I need help making the decision whether to put a new transmission in this truck, which I've become attached to, when the cost will probably be more than the worth of truck. There are very few junkyards in this area and I've heard used transmission for this model are hard to locate.

The truck is a 1997 GMC Sierra, 2WD, 5 speed.
I bought the truck used about 2 years ago. When a mechanic checked it out he said everything good except for a leaking rear seal along the shaft -- not sure that's the correct terms for it. Guy I was buying the truck from said, 'i'll fix that before you buy it.' I assumed he did (maybe he did and the seal just went again, I donno). I never saw any leak cause it was spraying all over the underside of the frame. Truck got harder and harder to shift (I was driving across country) but I thought it was just me cause I only ever drove compact cars before. Finally it popped out of gear on a hill in West Virginia (I started out in New Mexico) so I stopped at the next mechanic and he replaced the seal, filled the fluid, told me there may or may not have been any permanent damage. I lived in Maine for a year after that took it to transmission guy there who said there is a bearing going out (it whines in neutral, goes away when you put it in gear) and seems hard to shift because the clutch is high, but he thought it had indefinite time left on it. Last august I drove it down to New Mexico. The seal went again, this time it was superhot and I saw drops under the car while I was filling up. Went to a mechanic immediately (I was in rural area) who said he didn't have the part and told me it was okay to continue driving down to New Mexico, not much fluid leaking. But now it continues to pop out of gear on a regular basis. I parked it on a slight hill once and it popped out of reverse and started rolling forward (slowly, luckily, so I could jump back in and stop it). I'm pretty sure that means the transmission is hanging on by a thread, even though, as I said, I'm not mechanically inclined.

I'm debating whether to put in a new transmission, if necessary. I really like the truck, I liked it immediately and that's why I bought it even tho I was looking for a compact truck, not full size, now it seeems like there's no going back and it's great having a full size truck on cross country trips. The things is transmission and labor will undoutably be more than truck book for. Most everyone I know tells me to sell it for whatever I can get and get out from under it. I don't have a trusted mechanic down here so I've had trouble getting professional advice.

Any advice would be appreciated.

Replace the transmission. Keep the truck.

You know the truck and that it is reliable. If you put the money in the truck it will pay you back in the joy of driving it. FIX IT