When I'm driving at 45 and up (approximately) I get this vibration in the truck and a resonating sound in the cab. It sounds like it coming from the back end. It only is there if I've got the gas down, but if I let it off or decelerate then it goes away and there is just normal road noise. There is full oil in the rear diff and it doesn't leak (75w-90 Redline). It only does it at the higher speed and you can't feel anything in the steering wheel and it definitely doesn't seem like it's coming from directly underneath me, but from the back of the truck.

Any ideas?

The backlash in your diff could be loose.

Might be the pinion angle is a little too high:
- http://www.4crawler.com/4x4/CheapTri...FixingProblems

Under load, the pinion will naturally tilt upwards and that may be pushing it into a slight vibration. Backing off the gas lets the pinion tilt back down and run smooth.

Check your u joints, when mine started to go out there was a noticable vibration and noise coming from them until they finally broke.

thought he said no vibes, but he is getting vibes. in that case my money is on U-joints as well

The backlash sounds kinda good b/c sometimes if I don't let off the clutch real smooth i get a small "clank". Correct me if I'm wrong or don't know what I'm talking about, but it could be tied into the vibrations.

Yes there are vibrations coming from the back end somewhere.

that clank is either sloppy ness from u joints or its the springs flexing from axle wrap.

NO. I don't like the latter response: axle warp. lol.... So how can I go about narrowing down what the problem is?

Read the web page I posted a link to earlier, has lots of tips on narrowing down the problem.

Yes, I actually just got done reading it. So it's good to go that fast in 4wd (well the front half) and also on dry roads? If so, then I think that will be my next move.

Is it fine to go 45-60 with 4wd engaged and the rear drive line removed?

Yes, it just drives like a front wheel car. Steering feels funny, so be carefull. let us know what you find. Make sure it is in Hi, not low range. Yeah i know you knew better, but I had to make sure to say it, because some moron would ask

Haha good point..... But I would agree on the Ujoint clank a little and the vibes to. Good Luck though.

Have a carrier??

no it does not.