Originally Posted by chubbs878

....You need 3 Ujoints and a centering-ball bearing which is the most time-consuming replacement. Over $100 worth of parts easy,.....

Originally Posted by number9

As said it's best to remove front shaft to check. If you have nipple to engage diff lock it helps so D2 still driveable in RWD if repairs take longer than expected. The spicer/centering ball can be a PIA to replace and extend repair time.
......

Originally Posted by batard

1) Figure out whether it's a driveline- or engine-related vibration. Is there any vibration at idle?
NO
Does it vary according to road speed and not go away when you take your foot off the accelerator?
Goes away when taking foot off accelerator.

2) Assuming it's a driveline issue, is there any difference in the vibration when turning at the speed where it occurs?
No difference while turning.

3) If there's no change in vibration when steering, tires and wheels become the next suspects. Balance problems are common. A wheel weight can easily fall off, causing a balance issue. Rotating and balancing tires is also not tremendously expensive.
4 New tires 3 weeks ago.

4) If balancing and rotating the wheels does nothing to fix it, it more or less has to be something driveshaft-, transfer-box- or transmission-related. This assumes that the balancing was done correctly.

don't have 300 miles on the tires, should i take them back to balance and rotation?

5) Assuming the transmission isn't the problem, we've isolated it to the driveshafts and transfer box. NOTE: If you have a CDL, remove the front driveshaft BEFORE spending any time/money on balancing the wheels. If you drive around on it and there's no vibration, you know it must be the balance of the driveshaft. Otherwise this is the point at which you should inspect the front shaft thoroughly. See if there are any welded-on weights that might have fallen off, or if the U-joints look crusty. Check the flange that connects the shaft to the transfer box, as this can get loose and cause a vibration. Get the shaft rebalanced, rebuilt or replaced as needed. The rear shaft can also cause problems, especially the rubber coupling joining the shaft to the rear diff. Inspect that for cracks/warping and replace as necessary.

Originally Posted by RawFaith

How do I know if we have CDL?

Originally Posted by batard

A couple questions:

1) Does the vibration seem to depend on road speed? I.e. if you take your foot off the throttle when it's vibrating, does it stop or continue?

Yes vibration disappears with no throttle.

2) What year is your car, and what does the **** on the high-low shifter look like? Does it have "diff lock" written on the **** of the shifter itself? You can also check if it has diff lock by trying to push it left. If it goes, it has it.

Originally Posted by batard

This is some good direction. You don't have diff lock, and it's unlikely to be wheel/tire related if it goes away when you take the pedal off.

Another question: does it shake at all when you're idling? If you rev it while idling, do you feel any difference?

No issues at idle or when raving it up.

Also, is there any difference in the vibration when steering left/right?

While at the speed range of driving the vibration is there even when steering.

And finally, I'd suggest inspecting both driveshafts. First, feel for play when turning the shaft by hand. There shouldn't be much, but there's sometimes a little backlash, which isn't something to be worried about. Second, feel for lateral play in the ujoints by pulling/pushing both ends of the front shaft and the front of the rear shaft. Third, feel for play in the splines of the shafts (each shaft consists of two parts, a tube and a rod that inserts into it, held in place by metal splines running down the rod/tube). Fourth, inspect the rubber coupling that attaches the rear shaft to the rear diff. This thing can get worn out and cause vibrations.