Bad vibration

[Crashgordon56]

'87 Chev 4x4 shortbed, auto trans., 4 inch lift, 35 inch tires. Any speed over 25-30 mph produces a very pronounced vibration seeming to be coming from the rear of vehicle. Two new u-joints on rear drive-line and new pinion seal for back rear-end. Work was done several months ago by a professional mechanic with no issues until now. Not sure what's going on here. Any ideas?

                                                                                  Crash

[bd]

Have you performed a visual inspection?  Is driveline tight?  How about the pinion yoke?  Are tranny/transfer case mounts secure?  Is there noise associated with the vibration?  Does it change with brake application?  Is vehicle tracking staright?  Does the vibration change with road speed?  Did you throw a wheel weight, bend a rim, or Bubble a tire?

[Crashgordon56]

I did a visual inspection and the driveline seemed tight. The transfer case mounts could be a possibility I suppose. The vibration gets worse the faster you go and gets real bad when you apply the brakes. If you keep the speed below 30 mph it seems o.k. . As far as I could tell there weren't any missing wheel weights. No bent rims.

[zieg85]

If it were me I would put the rear axle on some "safe" larger type jack stands and really, really, and did I say really check to make sure it is safely on them.  Pick an area that if it should vibrate off the stands you could react to safely stop the truck.  Have a trusted person run it to where it just starts to vibrate and observe.  Remove the rear wheels and bolt the drums on and run it again, that should rule out a bent rim/bad tire.  If you can't move the pinion up and down and the u-joints are good something else may be wrong.  Can you rev the engine with the transmission in neutral and it is ok?  I have a vibration in my C30 that I think is my actual transmission itself but I have yet to verify.  Be careful!!! and if any of what I have suggested makes you uncomfortable, take it to a trusted mechanic and get his opinion.

[Crashgordon56]

Yeah whatever this is it did come on all of a sudden. When this first happened going down the road I did put it in neutral and rev the motor. The vibration is definately not in the transmission. I know for a fact it is not a bent wheel. I was able to limp the truck back home for 30 or so miles. As long as I didn't exceed 30 mph it ran o.k. . I'm going to see the mechanic who worked on the truck and get his opinion. He's only 1/2 mile down the road (lucky me!). Anyway I'll post the results when I get any!!!! Thanks.

                                                                           Crash

[thirsty]

Two new u-joints on rear drive-line and new pinion seal for back rear-end.

If you can't find anything obvious...
Before I did anything I would look at the new u-joints. Take the shaft out to inspect them. Then look into bigger problems like the pinion and such.

Sounds like you are describing a cheap u-joint that is taking a crap to me. My $.02

[Crashgordon56]

O.K. check this out! Just got back from the mechanics shop ( he wasn't there). Managed to get the truck up to 55 mph and guess what??? No vibration!!!! Apparently the vibes started after the truck was warmed up. I had driven 30 miles away from the house when it happened. So.......... the only two things I can think of that would be affected by temperature would be the tires and the rear-end. I did remember one other thing that may or may not have caused this. Before the trip I had set the parking brake for a reason I can't remember now. When I went to release it it seemed to hang up. If I had a rear brake dragging for 30 miles would it heat up enough to cause this problem?

[bd]

Why don't you pull the drums and do a thorough inspection?  If the oil level is good, chances are that your diffy is OK.

[Bitzer!]

I would swap the front wheels to the rear and vice versa. Sounds like an unbalanced wheel if you can drive through it or just get them all balanced if it's cheap enough out there.

Andy

[Crashgordon56]

The tires do need to be rotated so that will be my next move. I'll check the brakes at that time also. Thanks.

                                   Crash