I kind of follow vibrations like this
tires: rotate/rebalance, visible defects
wheels: check for wobble, visible dents/dings/bends
u-joints: check with driveline out
driveline: takes a shop & machine to check balance or re-balance
pinion: check for play, leaks, metal in diff fluid (hard to judge externally otherwise)
I would rule those out before jumping to transmission
BUT.....you never know, my TC was going out but I only had symptoms of some "jitter" that's what i call it at around 30-35 mph, no sound or other symptoms could mostly feel it when turning, kind of felt like maybe it wasn't wanting to release out of gear like a light banging feeling, def not rumble strip
you could maybe try driving it in 1st gear only and/or get it up to speed and put it in nuetral, then coast and see if there is any change or if the feeling/sound remains
also inspect hubs/brakes.