I don't know what year truck you have but I had a problem just like that in my 87 Blazer. We where driving in the mountains and we had this big bang happen. I thought someone had hit us from behind. We never knew what it was untill 3 weeks later when it happen again. This time it felt like it went out of gear. I took it to a local tranny shop and the guy knew just what it was. He was under the truck for like 10 min and it was repaired. There was a pump in the tranny that has a small gear that turns it. He showed me the old gear and there was 2 teeth missing off it. Every so often the gear would slip and stop the pump and it would go out of gear. The pump was on the side of the tranny I think and had 4 bolts holding it. He just pulled it out and put a new gear on it. He charged me $10 bucks !!!