This came up in another forum for a Frontier with vibration issues, but it's worth posting here for those who don't know about the tool. It's called a sirometer:

The little yellow loop on the lefthand side is a spring that extends as you twist the dial and will make a circular motion in response to whatever vibration frequencies are present. The stronger the vibration, the bigger the circles. You hold the device on the dash or wheel and move the dial until the loop makes the biggest circle, then read the frequency in the window. Knowing the speed of the vehicle and the rolling diameter of the tires, you can work back through the ratios to find the source of any vibration in the drivetrain.

Let's say your ride has a vibration that's worst at 50 mph and your tires' rolling diameter is 24". First, turn the rolling diameter into circumference by multiplying pi times the diameter. In this case, 3.1415 * 24 = 75.4". Then we can calculate the tires' spin rate at 50 mph using the formula F = 5280 ft/mi * 12 in/ft / 3600 secs/hour * 50 mph / 75.4" circumference. Multiplication and division are commutative, so you can do the operations in any order. In this case it solves to 11.67 Hz, so if the sirometer reads about 12, we know the vibration is from the tires or something else directly on the axles. If it's higher than that, we can factor in the rear end ratio. Let's say the rear is 3.00 reduction. The driveshaft will spin at 11.67 x 3.00 = 35.01 Hz. If the sirometer says around 35 then we know the vibration is from the driveshaft or the transmission output shaft. If it's still higher, you can keep working forward through the high gear ratio all the way to the engine. For those of you using metric, just do the rolling diameter in cm, then F = 1000 * 100 / 3600 * kph / circumference. It takes a little math, but it will also completely remove the mystery from any mystery-vibration. If you get a wacky number that doesn't seem to correspond, then you know it isn't in the drivetrain. Then you can investigate other sources like suspension links or front-end caster that may cause oscillations at a unique rate.

To my knowledge there are only two German companies that make the tool, but since Tecumseh uses the Treysit version as their official "small engine tachometer", they're not too difficult to find. Here are a couple of eBay listings:

https://www.ebay.com/itm/265316090753
https://www.ebay.com/itm/264580371612

I got my first one for free when I took Nissan's NVH course (Noise, Vibration, Harshness), but it's invaluable and I wouldn't be without it. Even if you don't do your own repairs, if you have a service manual for your ride and can do a bit of high school math, you can visit the repair shop already knowing what section of the drivetrain is vibrating and save paying for a lot of investigation in the wrong places (not to mention possible wrong repairs). The only thing needed is to figure out your tires' spin rate at the speed where the vibration occurs. Once you know that, the sirometer will lead you right to the most likely culprits.

Hope this helps you. Happy motoring!