You probably already have but it would help to understand "bullet proofing a 6.0". There are 2 camps on that motor some love it some hate it. It does have some issues which can be addressed.
Good article from someone much smarter then I...
I do - use reese hitch ball grease. Just a little dab on top o da ball and done.
Just reduces friction but really only needed for heavy towing applications. Just slows down the wear on the coupler.
Is it an E4OD?
I had one which exhibited the same thing. Ran it that way for a while just like others have said. Sucks but it had to be rebuilt - after that no problems. The AAMCO shop in Gainesville, Ga did the work.
My experience has been very good with the VG30 motor. Strong and dependable. 3.5 is a different motor but my bet is that it is a very good motor. Good Luck!!!!!