Toyota Repair: Vibration in 4Runner during driving, axle bearings, mechanic friend

Question
My '92 4Runner experiences a lot of vibration when I step on the gas above 60 mph (especially b/w 65-70 mph).  I took it to a dealer and they replaced the spark plugs and distributor cap/wires, which did not help at all.  I had the tires balanced and rotated and that still did not help.  Just recently some weird noises (sounds like it's caused by axle or wheel rotation) and vibration occurs intermittently at low speeds as well(20-30 mph). Since it only seems to occur when I am on the gas, could it be a transmission problem?  In park the engine revving sounds fine.  My mechanic friend said it could also be the flywheel?  I would appreciate any suggestions and estimates on repair costs for a problem like this.  Thanks.

Answer
Hello Allen, welcome to my expert page.
If there was a problem with the flywheel you would get the vibration also when the engine is reved up but this is not the case, as you said.The vibration seems to be related to road speed, there are two areas that need to be checked, the rear axle bearings and the driveshaft, an out of balance driveshaft can cause a vibration and it is the most likely cause, but noise is normally created by a bad axle bearing.