2007HarleyDavidson

After towing some small enclosed trailers all summer what once was a small short vibration/shutter is now becoming much worse. When at a stop and then starting to accelerate the truck shutters and vibrates for a second or two and it is extremely noticeable, it makes the change in the cup holder jump. I just changed motor oil and while I was under the truck I noticed the front differential leaking where the drive shaft connects. Truck has 94500 miles and a leveling kit with 20" rims and 33" tires. I'm doing the struts and shocks next week and would like to fix this problem at the same time. Does the vibration come from a bad Diff or does it just need new gaskets and fluid change? Or could it be something else?