1996 GMC 1500

I have a 96 GMC 1500 2wd with the 4.3L v6. When I come off of a stop sign and give it about half throttle the truck shakes really bad right before shifting into second. I checked the ATF and it was a little low but adding some did not seem to solve the problem. The shaking only occurs when giving it somewhat hard gas off of a stop sign and all the other gears shift fine. I also note that there is a thunk when I shift it into reverse. The shaking seems to be in the front but it is hard to tell. Could this be something as simple as a u joint or do I have bigger problems?