Hi all, obligatory "first post". I've had my 06 V6 4x4 Limited for a year and a half now. It's been great, I love the truck and it's easy to work on. It's not without issues, but I'm fine with that on a 15 year old vehicle and part of the appeal to me is that it's easier to learn on and fix up than a newer car!

Anyway, it has ~110k miles on it now and I've noticed in the past month or so that it's not getting into OD on the highway. Every other weekend I drive approximately 3 hours on Rt95 but lately it's all been in 4th gear at like 3200rpm. I understand this isn't terrible to do, but I'm worried about the cause of this issue. First time I noticed it, I pulled off and restarted the truck, and then OD worked for the rest of the drive. Since then though, no OD. I've searched around but haven't found anything definitive, but here's what I've learned:

• The tranny won't engage OD unless the engine and/or ATF temp sensors register a high enough temperature
• A faulty solenoid could be preventing the OD gear from engaging
• The AT fluid in my 06 is "lifetime" and the transmission is sealed, but possibly low or old fluid could cause this issue
• I don't think it's a "transmission" issue as much as an electrical or fluid issue

So, I'm trying to start with the easy and work to the hardest in troubleshooting. The V6 doesn't have dedicated "4" and "D" shift notches, so it's in the right gear and I'm not accidentally bumping it out of OD. The weather is warm and I'm driving for hours, so it's not a legitimate cold tranny issue. All other gears are working fine, the transmission shifts smoothly, and it doesn't hunt for gears.

First, can anyone tell me if I'm on the right track?!?

Second, can anyone confirm which temp sensor(s) the computer uses to determine when OD can engage? I'm thinking this would be the easiest fix. My engine temp gauge still works so I'm assuming that sensor is fine? I would be able to replace sensors on my own if anyone can share part numbers and a brief writeup.

Can anyone comment on the "lifetime" AT fluid? Would that be an issue at 110k miles? If a tranny fluid swap is known to help this issue, it might be worth taking it to a dealer for that; I don't think I would attempt that work on my own.

Moving on to the solenoids... I'm pretty hazy about the inner workings of a transmission so I'm not gonna pretend to know. Would a sticking solenoid prevent a single gear from engaging like I've described? Is it possible/worth it to check on or replace a solenoid?

I do my own oil changes and installed my lift on my own but I'm no mechanic. I do have access to a shop though so with info and a plan I can do simple repairs.

Any help is appreciated!