This will test you... drive line/tire/Susp issue? I am stumped

[232 thumper]

Hi All -
I have a 2000 silverado that really has me stumped. This weekend I had it loaded up for a show - big drum set, but proabably not more than 5-600 lbs. This was the norm for years up until December when my daughter was born and I took some time off.
As I approached 65 mph I got a vibration - but not like what you throw a wheel weight - it was like a harmonic vibration. I thought I had a flat and pulled over. nothing. Everything looked great. Hit the gas station and checked tire pressures - all were close to the usual 35. I hopped back on the freeway and the truck is as smooth as usual - until 65 - and it does it again. If I go over anything that compresses the suspension, it gets louder. It is definitely rotational - it is based on the rpms of the wheels - not the tranny/motor. So, The following day I head into Discount Tire to get them rotated and balanced (it was time to do so anyway). The problem is still there.

Here is the kicker - I unlaoded my truck on Sunday morning and headed out to run some errands - it ran flawlessly - not even a hint of the vibration. In late February I had my truck into the dealer to make sure that everything was ok (just a check up). The drive line was in perfect shape. Had all of the oils/fluids changed. So, I am leaning toward tires. Has anyone experienced anything like this?
My tires are 285/60/18 Nitto Trerra Grapplers. They have close to 70k miles on them - but they are in great shape. I really cant believe how well they have worn and how good the alignment on my truck has been.
I am very perplexed.
Of course the tire place wants me to drop 1000.00 on new rubber. The dealer wants me to load the truck back up and drop it off (at 50.00 per hour). I am just trying to understand what could be going on before throwing a ton of money at the issue.

Any thoughts are HUGELY appreciated - thanks!

[rjeffery]

A friend of mine has the same tires in 305/?/? on an obs 2dr. Tahoe. The same thing happened when we were on the highway at around the same speed. The damn thing vibrated and wobbled so much, I thought we were going to roll over. I don't know if it still does it, or if he ever found out what caused it, but it might have something to do with the tires.

[Rugerlab]

I got some of the same tires 285/75/16 on my 99 and it does it around 55 mph as well. I never had the shake with my old tires. It showed up after getting the tires, but i replaced my shocks and torsion keys at the same time so i didn't know for sure if it was the tires or not. But it looks like it might be now that i know others are having the same issues.

[GMMerlin]

When you loaded your truck, you changed the working angles of the driveline, this can cause a vibration.
Also I have had vehicles that will have a harmonic vibration from the tires when they have a large payload on them

[Sonny01]

I get a vibration at about 70 if I'm empty...put any weight in the truck and it rides nice and smooth...I think it's my Bilstein HD shocks.