Seems a bit high to me, but it all depends on what going rates are in your area. If the trans is well built by someone reputable and has low miles, maybe. Are the shift kit and the shell the only modifications? 1/2 ton axles usually aren't worth much, but paying for parts and someone to re-gear and install lockers on your axles can easily run 1500-2 grand. Some perspective-mild 700r4 rebuilds from a place like Bowtie Overdrives, Monster, or Mad Dog start at around 1200 bucks, and with that you get a lot of piece of mind. A stock rebuild could be even cheaper. Axle prices vary widely, but someone upgrading could very likely part with their modified 1/2 ton axles for a few hundred bucks. The deal may be the right choice if everything is in good order and perfectly aligns with mods you were planning on doing, but I definitely wouldn't call it a steal.