My truck seems to shutter a bit when shifting from 1-2 and sometimes 2-3. Also, when i let off the gas while traveling around 40, then hit the gas again to accelerate, i just heard/felt a loud clunk.

Is this my U joints, or my tranny?

Also, what is the correct procedure for checking tranny fluid? Some people say with the truck running in N, some say in P, im confused.

 

I would suspect your tranny, if it was the U-joints you would notice a huge vibration. As far as checking the fluid, I have always done it running at temp in Neutral. But for the final say, check your operating manual.

 

neutral or park doesnt matter just make sure its operating temp