Anyone ever had a pulsating vibe at highway speeds?

[Chevy3+3]

It may just be the nature of the beast but my truck has a faint pulsating vibe only at highway speeds. THe vibe or slight rumble comes and goes every few seconds. I can usually start to feel it around 60 mph. My truck is an 88' 4x4 crew dually 454/th400 & NP205 with 4.10 axle ratio.

Here's the funny thing.....About 5 years ago I owned an 87' GMC 3/4 4x4 w/350/th400 4.10 axle ratio. I can't recall the t-case model but it was a factory t-case. Anyway, the 87' had the same exact same pulsating highway vibe around 60-70mph. When the trans went out I had it rebuilt and the vibe was still there after the rebuild so I assume its not the tranny.

Anyone else ever experience this. I always thought maybe a t-case vibe on the 87' but now since I have it on my 88' crew it makes me think maybe its an axle or driveshaft problem since the t-cases are different models but the vibe is identical.