I have a 2007.5 GMC Serria 2500HD LMM with the 6-speed Allison Transmission 4 wheel drive, the other day I went and picked up a mini excavator and was towing it home when I noticed something iv never noticed before. I stopped at a red light in town and when the light turned green I let off the breaks and pressed the accelerator. As the engine began to rev up to start moving from the stop, a loud clunk and then a jerk happened as I started moving. It only happens when I'm completely stoped and then start to move (1st gear only) when I have a heavy load. It doesn't do it if I dont completely stop, as in if I let it roll a little at like 1 or 2 mph and then go. It also doesn't happen when I'm unloaded or towing a light weight utility trailer and lawnmower. Would this be caused by a slip yoke needing lube or maybe the trans. Mount bolts being loose?