ostranderbrown1

Hopefully someone will have something to offer ... I think I'm going crazy. I have a '91 k2500 with 24,000 original miles on it. The issue I'm having is there is a vibration and a thrumming noise between the speeds of 30 and 50 mph. Here's the catch: It only does it when the ambient temperature is around 80 degrees or higher (the warmer it is outside the worse it is), or the truck is warm and the vibration instantly goes away as soon as you pull the 4x4 shift lever into 4 high. As soon as it goes back into 2wd, the vibration and thrumming comes right back, usually worse.
Maybe someone can steer me the right way ... my best theory I have going, (although I'm not extreamly knowledgable on the subject) ... it is my understanding that the 4x4 system on this model of truck is vacuum controlled ... so I am guessing there is a vacuum leak somewhere that is made worse by warmer temperatures that is causing the 4x4 to partially engage ... is this a possible theory, or am I way off base? I had someone ask me if I had had the recall work done in which the vacuum system was converted to electronic ... I didn't pay much attention to the guy at the time, he had a '94.
I have the 5.7 liter engine ... the transmission (which is the 700-r4) was just professionally rebuilt less than 200 miles ago due to a bad torque converter. This problem existed before the issues with the transmission, and is exactly the same after the transmission/torque converter rebuild.
Thanks to anyone with any input,
~Cdawg

2wd-K5

universal joints? it doesnt explain the temp part though, if it were a vaccum problem, it just wouldnt work, it wouldnt try to engage while driving