96 Sub -- Rumbles at high speed

This is a K1500, 5.7L, 4x4, 170,000 miles

When it gets above 50 or so there is a low rumble vibration you feel in the seat or floor. Not in the steering wheel so much. Began to do it after a long trip we took recently. Slow to 40 or 50 mph it seems smooth.

No idea, but thought it was tires needing spin balanced which has solved vibrations in the past. They're old -- bought 'em 40,000 miles ago from TiresPlus w/ free lifetime spin balance. Far as I can tell, they still look good with lots of tread left (Dunlop Radial Rovers 245 75 R16). Just had TP do another spin but no change. No apparent dents in the alloy rims.

Before this situation, when tires were balanced, Sub ran smooth as butter even at high speed.

Any thoughts. Tires? Something worse?

THANKS - Steve

You may have some worn out axle bearings. Also check the engine and transmission mounts for wear. Then check your shaft u-joints for excessive play. Finally, if the rear driveshaft has a carrier bearing, it may be worn.

Bad U joints can cause strange things to happen, from high speed rumbles to vibrations only while slowing down or braking and then there is the obvious clunk that happens when first shifting into gear

Try this to determine if it's engine or drive line related: Take it up beyond the speed that it starts to rumble and shift the tranny into neutral and let the motor go to idle. If it's still there and you suspect tires, try to find a buddy who will swap tires and rims with you for a test.

Try this to determine if it's engine or drive line related: Take it up beyond the speed that it starts to rumble and shift the tranny into neutral and let the motor go to idle. If it's still there and you suspect tires, try to find a buddy who will swap tires and rims with you for a test.

Thanks for all replies. Had my mechanic test drive. Dropped it into N. Said 99 percent sure tires afterward. Some uneven wear despite good amt of tread left. Will see.