You may have a bad hub/bearing assembly. If you can rock the tire from side to side and back and forth w/o anything else moving, it's probably bad. Take the tire off and inspect. Once the wheel is off grab the hub and give it some good jerks up and down and side to side. It should be tight, if there is slop at all replace it.

 

i've read that the loud thud when disengaging the 4wd could be caused by a bad U-Joint. is that really the case? cuz then it would make sense. I get a LOUD thud while disengaging, and while engaged the front end shackes like mad.

 

Are you moving when you engage/disengage? let me know when you check that front u-joint...seems to be the next step in your diagnosis.

 

moving or stationary. there is almost no sound when it engages and a huge thud when disengaging.

front wheels spin when i hit gas but the front end shackes like crazy at anything faster than 40mph

 

Make sure before you take out that front driveshaft from the front yoke that you mark the shaft and yoke with a sharpie or crayon so you put it back exactly how it was, that will keep the balance.

 

lol i hardly know what those terms mean, not very mechanically inclined.

could you guys tell me what to ask the mechanic to look at next time im at the shop?

basically when engaged the front end shakes alot, when disengaging there is a huge THUD coming from under the car.

prior to this happening it was the other way around. first it would be a thud when engating and like a clunk sound when disengaging

i forgot to mention that just before this happening i was going through some deep and frozen snow. could that have effected anything?

 

Some noise is normal when engaging/disengaging lots of parts moving. The other thing to check is see if you have a bunch of snow packed up in the front end including the wheels/driveshaft etc. Packed snow/ice will actually cause things to get out of balance and vibrate.
If you're not comfortable fixing it yourself, you'll have to take it somewhere. If it is the front u-joint, they're only about $25 U.S. and less than an hour to change. Should be in and out of the shop for $100 or less (if that's the problem).

 

yea, after all this snow were going to take it up to a local mechanic and let him know what you guys have suggested. Thanks everyone.

 

So can this cause any more damage? If I just keep in in 4 wheel under 40 it should be fine right? I just can't get to the shop till mid Jan and I only take the Blazer when It snows or I have to tow so It doesn't get used much. I'd just hate to make it worse or have it sieze up or something crazy on my way to work in crappy snowy Ohio roads.

 

Honestly, it's hard for us to make that call w/o actually driving the vehicle. You do run the risk of making whatever it is worse, or even stranding yourself if something breaks.