Had my Jimmy in the shop the other day to get my high pressure line for my power steering replaced $170 including labour and $40 for a front end inspection, and also got him to figure out what the TERRIBLE rattling sound is.

anyway he says i need to replace both bearings, says he could more 1 tire an inch or inch and a half each way, sounds bad ya, i plan on picking up the parts this week for that, and doing it with my back yard mechanic dude.

but heres where i need help, i have a baaad tinny rattle thats very speed dependant. sounds very similar to when the inside of my cat converter busted up and rattled around. only this seems to be coming from closer to the front of the jimmy. I have to be going over 35-40km per hour for it to start. very intermittant and anything i do with the steering/gas/break dosent seem to affect it at all.

replaced my muffler yesterday, that was $200 for nothing, grr, thanks T.T auto for a false diagnosis.

i was laying under my truck for at least an hour and alls i could find were 2 loose bolts on a skid plate but if i shake it, its not the sound im looking for......everything is tight....and the front end inspection came out clean beside what i stated before. everything i can get to is greased, lower ball joints are now as quiet as ever.....im sooo stumped.....

but to me, it seems like its the passenger side tire

 

look at the backing plates in back of your rotors. they rot and start to scrape the rotors (I just pull 'em away a bit) if your bearings are bad and lets the rotors cant it seems like even more likely. just a possibility

 

Could also be broken heat shields on the exhaust manifolds on back.

 

You said you replaced your muffler, have you checked your converter?