I'm with the 'stick With It' side, ok a definite sickener having to replace the gearbox no question about that but look at it this way the original box done almost 100k so depending on your mileage if the new box does the same how many years driving are you looking at.
Service items don't really count and you probably wouldn't even consider them had it not been for the gearbox issue.
Front diff? again how many miles has it done already, if it's the original diff treat it like the gearbox, once rebuilt it should be good for a fair few 1000 miles.
Ok so you're looking at an 08 pick-up that's really cost you 9k rather than 5k but from here on in (after a year of ownership) it's going to cost every year you own it and in that it's no different to any other vehicle used or new. You either spend on maintainence or you lose in depreciation. Also since you bought your's every other truck out there that's newer than yours has had a years extra mileage on it since you bought yours and each one is getting closer to having the same problems that you've already dealt with.
Finally, remember if you sell it because you're sick of it that your 'Old' truck is going to be the next guys 'New' truck and he'll be delighted with it (as you probably were when you bought it although the previous owner probably had lost interest in it) especially as it's had a new gearbox and a rebuilt front axle.
I've been the guy who after the initial 'Love affair' with 'my' new car got fed up with it, didn't look after it, didn't wash it, let rubbish gather inside it etc etc, sold it and next thing you see it in the hands of the new owner and it's clean and had a bit of polish and he's pleased as punch with 'his' new car.