Intermittent Vibration/Wobble

Check the drive tires,tourqe ARM bushings I like to drop the driveline and check the needle bearings they might seem tight then you drop it and the inside of the cap and yoke are chewed up from lack of grease

 

Check ride height on rear, steer tire balance and for out of round, cab mounting and suspension.

 

The above posts are bang on. Any new work done? Tires? Sounds like a leveling valve/ ride height issue to me. Do you feel it in the stick?

 

Cumming through the stick makes me think it's in the tranny or driveline. Could even be as far away as the rear diferential. I had a T600 with a Super 10, and for a loooong time had a rattle in the shifter. You know one of those that don't rattle anywhere else but where it wants to rest naturally going down the road !!!!

Anyway finally cooked my rear diff cause it had literally rusted through in a couple places to where the oil would slowly leak. Well with 4 other trucks to keep track of,you can figure out what happened. So I replaced the rear diff and the first thing I noticed after doing the job and taking her on a test run was that darn rattle in the gearshift was gone !! So your problem may be treansmitting through the driveline. IMO !

 

There is one more area which 99% of people miss and can damage not only engine, but transmission, drive-line and input shafts as well. It's as simple as that crankshaft damper bolted to front of engine which all fan belts originate. While 520,000 miles is relatively a young engine, fact is these dampers must be replaced every 500,000 miles. These dampers do and will fail and resemble a wheel or driveshaft out of balance. These are fairly easy to replace but something which must be on your maintenance radar. You will not know when they fail, but if you simply have them replaced every 500,000 miles you'll never have to find out when they quit working but when they do fail, they can cause many thousands of dollars in damage to engine and drive train. They only cost $500 - $700 dollars and are worth the investment for long running trouble free operation. A failed damper will snap a crankshaft, and can snap a drive line input shaft. It mimics a bad u-joint and you will feel it in the seat bottom. Just something I want all mechanics and owner-operators to think about.