Rear Main Oil leak on M1009. Could the oil cause shifting issues?

During a 500 mile Road Trip I experienced what felt like stuck-in 2nd on the freeway. The sound just did not seem high enough, and there was a floor felt rumble. I thought I might not yet have shifted into 3rd. After sever miles, the pitch went up and, at least felt like 3rd.

Any reason to believe that the rear main leak could affect 2nd-to-3rd shift issues?

Hose inspection will be done Friday in case the heat from the engine is collapsing oil-soaked trans modulator line?

Anything else?

No the engine and transmission act independent of each other. Unless the rear main seal is the transmission rear seal. Then yes a leaking seal would effect it.

So, the transmission seems to be the issue- separate from the oil leak. Ok.

Over the years I've heard people with GM transmissions say "Oh, they can go in and tighten the bands," when talking about "slipping" transmissions. I've never had a transmission shop ever tell me that's what was needed. I always end up paying for a rebuild. Is the trouble I'm having getting into 3rd gear something that can be fixed without a new transmission? Could it be the VRV? or something else I can adjust?

I have to ask because I've never torn into a transmission. Really.

Hard to say without being in the truck with you. Have you checked the fluid? Is it low, burnt, or looking like metal flake paint? Is your kickdown switch stuck? Vacuum modulators are cheap. Have you tried replacing it? They can fail and suck fluid into the vacuum line. There’s an adjustment screw inside the port on the modulator that can be tinkered with as well.