stuck transfer case?

Hey everyone. New to the board and it looks like a great place for info. I have 96 Yukon 5.7 litre Vortec 4x4 four door automatic. The fuel pump and connector went bad within a week of each other and i've replaced the fuel pump, twice as the first was a bad one, and the wiring connector to the pump. I also replaced the plugs, cap and rotor and the fuel filter. Ever since my mileage has been cut in half. I used to get about 750 km highway and abour 650 city on my 115 litre tank. now I get about 450 highway and 400 city. The only thing I can think of is that the transfer case is stuck in four wheel drive but the front wheels aren't getting power though. When I put it into four wheel drive it feels like it always did and all four wheel spin. I can remove it from 4x4 and it feels like it's back in 2x2 but it seems to feel like it's struggling against itself. I had the same thing happen in a 92 jimmy. So to make a long story short is there a switch I can test/replace that signals the transfercase to go into and out of 4 wheel drive? Getting a humming noise from back of truck. And I did check that the parking brake isn't on. Any help would be great. Thanks and Happy New year.