Sierra 2500 Drvln Vibe

[Spurshot]

I have a 1999 1/2 New Sierra 2500 with a 6.0 eng, 4L80, trans and pushbutton NP246 transfer case 210,000 miles. I noticed a vibration in the driveline after replacing U-joints. I'm not saying it wasn't there before, just that I noticed it afterward. (added) No lift or any other modification to this truck and driveshaft angles checked by a 4wd pro.

 

The vibe occured in 2 wd at speeds above 45 mph and became pretty significant at freeway speed, enough to buzz the mirrors.

 

I isolated it to the front driveshaft which was turning while in 2wd. I did this by jacking the rear and removing the wheels/tires to run the rear driveline without wheel/tire vibe influence. During this exercise, I noticed the front driveshaft turning while in 2wd with the front wheels/tires on still on the ground.

 

I pull the front driveshaft and the vibe is gone. I wasn't aware that the front driveshaft should be turning while the TC is in 2wd, but others tell me it turns in 2wd by design. I still don't know if that's correct. The front driveshaft can be turned by hand while in 2wd, but the drag of the internal TC clutches is significant.

 

BTW, all the 4wd hi and low fuctions seem to work fine as tested in the dirt. 2wd seems to actually be 2 wd when driving.

 

I've chased this problem for a few months and couple thousand dollars so far. Here's the list of things done to date:

 

New TC rear DS yoke bushing

Replaced the new Jap autoparts store U-joints with Spicers & checked for free movement

Check front DS for straightness, balance by a DS pro

Rebuilt front diff, especially conscious of pinion bearings

Rebuilt TC with new clutches, chain, bearings etc.

 

After the rebuild of the NP 246 TC, the vibe is much reduced, but still noticable in the pedal and gets noisy at around 75 mph.

 

I have not as yet tested for DTC fault codes.

 

Questions:

1. Should the front driveshaft turn while in 2wd?

If not, could this be an encoder/motor issue of not properly/fully placing the TC in the correct position?

 

Even my 4wd pro and 4wd enthusiast brother are at their wits end on this one. Help!

 

Chuck