Transfer case fluid
 

What can I substitute for 75w gear oil?

All depends. What does the book call for? What year, what transfer case number, which engine.? Help us help you.
I would suspect that 80W-90, or if you can find it, straight 90 weight. The things to consider are the normal operating temperature range, and what the FSM specifies. The really important thing, from what I understand, is the GL- rating. GL-4 or GL-5. I believe the transmission (if it's a manual) and transfer case need to have GL-5, and the differentials need the GL-4, but I might have that bass-ackwards. Terrible memory.

Does that help any?
Pat☺

2017tacoma 4x4 auto.

Ok, all depends on the temperature range you're going to be seeing. Only using it in the snow and ice? Lower weight oil is better. 80W, or 80W-90, something like that. When I lived in Yuma, AZ, a VERY hot place, I used 90W-120 in the transmission and differentials. It worked great.

Most important, through, is the GL rating. GL-5 for the transfer case, GL-4 for the differentials.

Does that help at all?
Pat☺