Need Help!!

I have a 89 k1500 350 engine. I just bought it a month ago. All of sudden I been getting loud metal on metal scraping, banging noises. My fatherinlaw used to do alot of work as a mechanic says its probably a u joint we went for a ride nothing happened. It seems that when the problem starts to happen the faster I go it goes away. and when im coasting still dont hear it maybe a slight ping here and there. it will happen when im going slow/starting off . I actually thought my exhaust fell and was dragging on the ground.. It had a sound like the exhaust pipe was smacking the ground. also does the noise in reverse. The truck is lifted about 2.5 inches. its an automatic. It didnt happen for the first couple weeks. I dont work on trucks for a living and dont know the best way to troubleshoot to find the problem. I just would hate to take it to a garage for something I can do. I gave all the info I could about the truck that would help any.

Andy

Where does the noise seem to be coming from? Possible rear diff problems--bearings or gears worn--how many miles on truck?

That could be I just thought about that on my way home today. When I stick my head out of the window and try to listen sounds more towards the rear. Truck has 220,000. Not sure what was replaced. if its not making any noise driving forward and I back up it starts up right away also... Thanx for the reply...

pull off rear drums and check brake shoes--probably dragging or something loose inside--if is dragging the drums could be very hot so be careful if you have just driven it