The quick and dirty, well maybe a little longer

Driving down a semi paved back road that turned into mud, took about 5 mins to get through. Nothing really deep about 1-2 ft or so. When I got back to town, I notice a little(very little cloud) steam coming from below the truck, had a quick look couldn't see much with all of the mud underneath. Cleaned the truck, washed underneath. Could smell something that smelt like burnt coolant, kind of had that sweet smell to it. Had a good look since it was semi clean and still nothing, no leaks, crack etc. Took it out for a drive later that night and noticed that the O/D button is not working when the truck is in drive, truck shifts fine, drives fine. Reach about 120 km/h and it starts to shutter. Didn’t notice any damage to the u-joints or drive shaft, or any missing weights.

What I was wondering is there a way to easily check the transfer case fluid level or is there a relief valve is the transmission and transfer case over heat. I highly doubt that it overhead, 5 mins max I was in the mud, even then traction wasn't a big problem and I wasn’t going that fast. In regards to the O/D is there a sensor/switch or something externally connecting to the transmission that might be lose?

Any help is greatly appreciated

As far as shuttering make sure you cleaned the mud off completely from the inside of the wheels and rims as well. Often that's the cause for shuttering or tires out of balance.

Thanks...I'll double check to make sure there is none left there, but the vibration seems to be right under the front seats, nothing more than usual through the steering wheel.

Talked to a nissan service rep, he said to check that that the temperature sensing switch on the transmission has not become disconnected or the wires are damaged, this would cause overdrive not to engage in drive.

Question, where is the temperature sensing switch located on the transmission? I went to five different places yesterday and couldn't find one single repair manual. I can order it online but don't want to wait that long.

It's inside it with the valve body etc.