Bad U joint or something worse?

Recently as I was heading to work I noticed what sounded like rocks being thrown up under the truck, as if I had ran through a small pile of dirt on the highway. As I continued my drive to work I started to notice ( only cause I had my window down as I have an 8 inch lift and 35's on it that roar like a lion ) that I heard some heavy ticking noises. It started out towards the front end of the truck, and gradually worked its way back, I thought, ok I piked up some road debris and its hung up wrapped around the drive shaft. As I left work for the day I drove home as I normally would and I started to notice it again, but it was louder. Sitting stationary it made no obscure noises at all, everything sounded fine as it would any other day. As I started to accelerate, I noticed a slight clicking noise, as I began to increase my speed, it became rather loud, but at times would back off to almost a whisper. This morning the same thing, went to work, it clicked like a kid had a baseball card stuck in his spokes, and on the way home after a good rain, it barely made any noise at all, until I got back into my city and started the traffic light shuffle. I did take notice to one thing, as I pulled out from the gas station and threw it into drive, I heard a clunk that sounded like it came from the rear end of the truck.

Could this be a u joint issue, or something deeper, like a rear end or transmission issue?

Thanks in advance for your response.