I have a '91 4X4 manual 5-speed pickup with IFS, manual locking hubs.
Previous owner geared it 4.88 for 34" tires. How can I tell if the front was re-geared?

The reason I ask is when the hubs are locked there's a significant vibration at/or above 45mph. Any faster and the vibration gets extreme. So much that the change wants to vibrate out of the ashtray and the dash sounds like its coming apart. Below 45 and it goes away-ish.
I'm wondering... if they didn't regear the front diff to match, if this would cause that vibration at higher speeds? or would the symptom of a gearing mismatch be more extreme than that (i.e. binding and shuddering as soon as the hubs were locked)?

Scenario:
We're in the middle of a pretty significant snow storm. My only mode of AWD/4WD is my truck. Driving to work was snowy and slow-going but when I left to come home the roads cleared up a bit so I took it out of 4WD but left the hubs locked. It wasn't till I got on the highway that I noticed the extreme vibration. It was so bad I had to pull over and unlock the hubs.
Driving in that morning I didn't notice it because traffic was moving pretty slow.

 

You have to match the gears if you do a replacement. I would be really surprised if they were not done. If the front was not done you would have other problems with the drive train wanting to bind up like a rubber band. As far as the vibration...check the front driveshaft for a u-joint going out. Does this have a lift kit?? Probably does with 34s --CV axle/s could be binding also or just not liking the added speed

 

it is lifted, yes.
I ditched the 34's for 32's shortly after purchase, along with replacing both CV axles.
I'll check the frond drive shaft when the snow melts. For now its undrivable with the hubs locked over 45mph, or at least feels terrifying.

 

Vibration when leaving the hubs locked but not in 4wd points to CV or driveshaft, so I agree with what mechkw says. Try locking one, driving, then unlock and repeat on other side. That should tell you if it is one CV or the other. If neither, then it's probably your driveshaft.

 

Oh, just read that you replaced both CVS already. Hmm, likely driveshaft then. I hate changing joints, just not easy for me for some reason.

 

It was common for large tires to develop horrible wear patterns. They were often an off-brand tire with aggressive tread. Was the same vibation present before you replaced tires?
Swapping tires with another vehicle is the best method to test, But depending on the overall wear you might be able to rotate front to back to see if the vibration moves

 

Tires are Falken Wildpeak AT3W. So they're pretty creme de la creme.
I've never ran this truck at highway speeds with the hubs locked before this past couple days. Rides beautifully when not locked in.

 

Can you shoot us a picture of the CV angle/s? Most lifts over 4'' get into differential drops or a flat out replacement with a straight axle because of this issue