Truck "jerks" at interstate speeds after 40 or 50 miles.

2003 GMC Sonoma 4x4 145,000 miles. The truck has recently developed a "jerk" when driving down the interstate. It is a hard enough jerk that you can feel it in the rear end of the truck (differential gears jerking). It only happens when driving at interstate speed for about 40 or 50 miles. Once it starts, it happens regularly about every 30 seconds or so. You have to pull over wait a minute or two and you can drive another 40 or 50 miles before it starts again. Over the last week or so it is getting more frequent (happening at a shorter distance 30 miles or so). I have checked the transmission fluid and it appeared ok. I have changed the differential fluid and I did not see any metal shavings in the old oil. The gears appeared to be ok (no obvious damage to any of the teeth). The driveshaft feels tight when I grab it and try to move it. I am not sure where to go from here. I initially thought it was in the drivetrain, but now I wonder if there is something electrical that is getting hot (but only at interstate speeds seems to eliminate that). Please help.