Some people have told me if the trans isn't acting up and you don't have to, don't change the fluid, same with the diff, my truck has 150k and shifts strong with no problems, the trans fluid has never been changed, and I'm afraid that putting in higher viscosity fluids could actually do more damage on worn out parts than the current less viscous fluid. Just wanna get a general consensus of what you guys think. Thanks!

 

Just do a pan drop and filter change on transmission. Will need 4-5 quarts of mercon v to refill. Change rear diff and make sure to use syn 75w-140. 2.9 quarts plus friction modifier if posi.
You can add a drain plug to your transmission pan to make future drains easier.

 

^^ Yup, I did this with mine, no problems so far. Actually runs better now than before.

 

Change it.