Please help identify this bolt and where it goes to

Last week I was changing my oil, laying on my back underneath the passenger side and undoing the oil filter when this bolt fell down on the ground:







Its a 17mm with a total length of about 63mm and it has a '6' stamped on the bolt head. I immediately checked all the steering and control arm components on the passenger side and all the fasteners there appear to be in their correct locations.

However, now during slow speed turning or gear shifts I got a persistent vibration from the passenger side that rattles the plastic on my passenger side dashboard.

I will be going down under the truck again this weekend to see where the hell this bolt came from, but if anyone knows offhand, it would be a great help. I'm not concerned about the noise, but a bolt of this grade missing out of something is a safety issue I need to address.

Perhaps trans. mount or crossmember? Motor mount?

 

Shit, went underneath the truck and I'll be damned if its not a transmission bellhousing bolt!.

Stuck it back in and torqued it to the spec from the manual - 53 ft/lbs.

 

Check all of them.

 

I checked the couple I had access to with the torque wrench and they were all tight. I put my hand on most of the rest I could reach and they at least werent loose to the touch. I have no idea how this one worked itself loose. I dont remember ever touching it and since Ive owned it, I have never pulled the trans or anything that would require the removal of the bellhousing bolts.