Seems after re gearing my 1992 ford bronco a vibration is occurring at speeds 40-50 mph and on and off the gas at any speeds. With stock gears and the 6 inch skyjacker lift there was no vibration, it happened with re gearing .Havent had time to check u joints , but was seeking opinions of others who have did a 6 inch lift and re geared if they had similar problems . Shop who re geared noticed the vibration and took it to a 4 x4 shop, in which the owner said the pumpkin would have to be realigned(cutting welding etc) opinions Please thanks TR

 

With 4.88 gears your driveshaft will be spinning faster than before. That would make a previously existing vibration worse. If it previously was barely (or maybe not even) noticeable, "worse" could be what you're feeling now.

Lifts will usually mess up U-joint angles which will cause vibrations. If you don't have a double Cardan (CV) joint you ideally want the angles at each end of the shaft to be the same, so you want your pinion parallel to your output shaft. If you do have a double Cardan joint then you want the pinion turned up so it's almost pointing straight up the driveshaft (usuallu about 1 degree low is preferred).

If it's the front axle, then yes, a lot of work is needed to change angles. But then again, if it's the front axle you might be OK just leaving it in 2WD and unlocking the hubs if your fourwheeling is at low enough speeds to keep the vibration down (that's my solution with a 6" lift in my '71 Bronco).

If it's the rear axle then all you need to cut and weld is the spring perches (and maybe the shock mounts). I did that on the rear of mine. wasn't a big deal.

 

Other options to explore might be adding tapered shims or shackle mods to aim the rear diff, and possibly "clocking" the transfer case for better front driveshaft angles.

Another note is that vibrations in that speed range are typical for out of balance or out of round tires. Which I'm guessing may have gone on to match the lift.

Make sure the driveshaft is clocked properly if it's a multipiece. Improperly clocked u-joints will destroy ALL KINDS of things. I doubt this is the issue since with a 6" lift you would probably no get the truck around the block with the d/s clocked wrong, but some driveshafts can be assembled half a tooth off due to an odd spline count, which will be much less severe than the 90 degree misclock that people err with now and then.