I have a 1984 Chevy k10 suburban, a couple weeks ago while driving the transmission acted as if it was slipping then seemed to be in neutral, if I place the shifter for the four wheel drive into 4high then it will drive a short distance then act as if it's in neutral, then if I put it back in 2high i can continue driving, what would cause it to do that?

 

What weight oil would go into a 76 muncie tranny? Do you know? Please and thanks