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I was going to change my universal joints since I am getting noise and vibration at 40-50 mph. I was looking in the haynes manual and it says to drop the transfer case?!?! I took a look underneath and I swear I can get to the top drive shaft without dropping the transfer case. Before I give it a shot I have to ask if you really have to drop the transfer case?
No. If you can get it out that's all you need. Mark the drive shaft and the yoke so it goes back in the same relationship.
It may not be the u-joints,mine did the same thing,replace u-joint twice,and problem still there,I'd be going 50 to 60 and it would vibrate and I'd lose power,couldn't figure it out,took to transmission shop,he raised it with someone in it and cranked it and ran it in the air,the parts on both ends of u- joint were bad.He sent it to a specialist and they straightened out,cost 200.00 and that was almost 4 years ago and it runs super.
I was underneath the truck yesterday and I tried to my hardest to get some movement out of any of the u-joints and I couldn't get any. I think the u-joints aren't the problem, but i'm not sure what it could be. I might just try to put up with it since I only notice it in that speed range. It doesn't affect performance any.