TCM codes P0700 & P0605 Help

[JennytheJeep]

My 2007 Cherokee KJ 2.8L Diesel (automatic) has been a headache since I bought it a couple of months ago. It won’t shift into 5th gear, seems to start in 2nd or 3rd, and is overheating, along with other issues.
Two local mechanics ran diagnostics but couldn't work on it, saying only the dealership could handle it due to part restrictions and code resets. So, I took it to a dealership hours away, and they said it’s too old for them to get parts, recommending a transmission rebuild instead. Here’s their report:

• Active faults in PCM and TCM
  • PCM: P0700 Trans control 3-4 solenoid
  • TCM: Ud pressure SW Sense circuit
  • Internal control module ROM
  • Ud pressure SW rationality
  • Flashing TCM internal fault
• Recommended removing sump pan to inspect for metal debris
• Suggest a transmission repair agent, as they can't get parts

A transmission shop estimated $6000+ for a rebuild without looking at it, which is more than I paid for it initially, and I’m unsure if that’s realistic or if they just didn't want to do the work. I don’t know when the transmission was last serviced, and without a history from the previous owner, I’m hesitant to change the fluid/filter in case it’s keeping the gears going as they are.
Any ideas on what I should try with this lemon (besides setting it on fire)?