Weird vibration at 50mph

[Flightnerd]

Hey, been looking around on here for awhile and couldn't find an answer. I just bought a '91 Ranger, extended cab, 3.0L 2WD, manual. Around 50 mph it develops a very low frequency vibration. Sounds like somebody put a subwoofer in the back. Above 50 it's fine, and below 50 it's fine. Some friends of mine have suggested it's the driveshaft out of balance, or a U-joint or the carrier bearing. Wanted to see what y'alls opinions were. Is this a critical problem? I'll be making an 800 mile drive in a few weeks to go home from college. Should I get this fixed before then?

 

[feellnfroggy]

Usually tire balance starts around 50-60 then goes away at speeds above that, check your tires. Also check your alignment.

 

[adsm08]

U joints usually happen at much lower speed, carriers usually have a wider range than that.

Get the tires balanced, it should be done every 20K miles anyway. For that narrow of a range and that low of a frequency my money is on rear end gears. Differential "boom" is often described as either a pressure on the ears or feeling like a sub. It's caused by a section of the ring gear being worn smooth. It's annoying, but not fatal.

 

[Flightnerd]

So my rear Differential is eating itself up? The tires were just balanced by the guy I bought it from two weeks ago.

 

[Flightnerd]

And it is very much like a pressure on my ears when I drive it. Going to try and take it to a driveshaft place next week and have them look at it.

 

[adsm08]

So my rear Differential is eating itself up?

That is far more extreme than what causes diff "boom". It's not that the teeth have been worn off. The surfaces of the gear teeth are rough when new. It allows oil to stay on them better. After so many miles the rubbing action of the teeth across each other can wear them smooth. It allows them to make a very low frequency sound.

I'd change the rear end fluid. It probably won't help your vibration any, but it may help prevent further rear end wear.

 

[Flightnerd]

ok, thanks for the tips