Please tell me why my truck wont climb hills

I have a 1989 suburban 2500 when it comes to climbing hills I can walk faster than this truck can go . It is fine on a flat highway no problem doing 80. It just will not go up. I have my foot to the firewall when I have to go up hill. I have checked the timing, changed plugs and wires


Now I need your help

Is this a toyota?

sorry I do not know how this got into the toyota side of things it is a chevy