My Brother has a 2000 Blazer. It has the push button 4WD. When the truck is in 4wd, there is a thumping that shakes the whole truck, this gets worse/harder as you are slowing down.The faster the truck goes, the faster the thumping, although not the strength.There is an burning odor associated with this also. When you shift back to 2wd, this all stops in about about aminutes time and everything is seems fine, butyou can hear athunking noise though. The truck was involved in a front end collision. There is no abnormal wear on the tires and the ball joints, shocksand tires are new.

 

I sound something like my problem. I dont know what the hell it is. Thinking about replacing mywhole front diff. Do you know if his 4wd work when he has it in 4wd?

 

Has there been any work on the axles? Sounds like there is a difference of gear ratios... You may want to jack up the truck and put it into neutral, engage the front axle (vice grips) and turn both front wheels one revolution with someone counting the revolutions of the driveshaft (mark the driveshaft at one location and the diff housing at 4 locations 90* apart). Repeat on the rear axle. You should get the same number (3.42 turns for 3.42:1 gears, 3.73 turns for 3.73:1 gears, etc). You get the picture. A slight mismatch in gear ratios will cause all kinds of havoc.

 

Yes, The 4wd does work.

I got a chance to look at this closer and saw that there was no grease in the front diff at all (he just bought it). I have added grease and the severity of it has gone away, but you can still feel it, it feels as if something is binding and then releases, that's when you hear and feel the thump.There is no outside visible damage on the case. I put a white cloth in the drain hole, used a pick to see if I could find metal shavings and there was none. I'll have to jack it up and do the marking and chalking when MN warms up next week, He's not in a hurry to drive it...yet.

 

By 'grease' I hope you mean gear oil. And if there is no gear oil in the front diff, I hope there was no damage...