How do you diagnose a bent axle?

[SUX2BU99]

What's a good way to diagnose a possibly bent axle? I got this weird oscillating vibration going on in my truck at around 40 mph. It's not a constant vibe but comes in and out, in and out, etc with speed and then goes away after 40 or so mph. Even if you take an axle out, how do you determine if it's bent if you can't physically see it by looking at the shaft?

[HAULIN IT]

Sux, A farmer way to do it is this: Jack the back of the truck up so the tires are off of the ground, set on jackstands, ect., get a jackstand, take the middle adjustable part out (or suitable solid item)...Clamp a piece of angle iron to the top sticking out (this will be your pointer). You can do it a number ways, but kill Two bird's with one stone...put the "pointer" up against the face of the bead of the wheel (not the very edge, the first flat area) so it is just touching & rotate the wheel carefully by hand & watch the pointer, if it touches the wheel in a spot, readjust accordingly & recheck...your goal is to have it just touching all the way around. If you see some variation in the wheel, take the wheel off & do just the axle, but understand the amount of error will look much less, a dial indicator should really be used at that point, much over .015 or so runout would be too much at the flange. Understand?    Lorne

[VileZambonie]

A dial indicator.

Check your driveshaft and u-joints first.

[Irish_Alley]

what about bearings in the rear i don't know what there called but the pinion bearings will cause the drive shaft to flop like a fish out of water. if I'm understanding you right it, at 20 or so no vib and 40 you have vib and at 60 you have no vib. if thats the case also check your tires for and bubbles from broken wires and mud in the rims also look to see if any lead weights are missing off the tire you should see a differentness in clean/dirty spots were the weights used to be

[SUX2BU99]

Thanks for all the advice guys. I'll probably do the cheaper and easier stuff first like have the rear tires rebalanced and then the driveshaft balanced. Once I get this rear end fixed AGAIN I'm going to get this stuff done just to eliminate items as much as I can. They replaced the rear u-joint and all the bearings in the diff when they did it the first time. I'm pretty sure that would include the pinion bearing but I'm not 100% on that. The receipt says "axle and diff bearings" but I'm going off memory about that. I didn't have this vibe before I swapped in this axle last summer.

Lorne, good farmer trick there. I believe I understand. Basically, a person just wants something fixed to the ground to point against a reference spot on the rim to see if there is movement. Making the pointer solid so there isn't any movement is what is going to indicate a wonky axle because the rim might move around against the tip of the pointer, which remains fixed.

How would a driveline shop do this? With the axles installed or would they have to remove them?