Worsening rumble in 2WD only on highway

This is a worsening rumble-strip-like vibration that starts at about 80km/hr on the highway and worsens if I try and go faster. It disappears if I lift completely off of the gas pedal and returns when I try accelerating again. Over time, it's been getting more and more severe.

Two different people have looked at the U joints and said they seem fine. One shop lifted the rear end up and discovered a "clunking" when manually moving the rear wheels between forwards and backwards. I can replicate this clunking by shifting into forward, moving, then shifting into reverse and moving again and vice versa.

378,000 km this truck has served me well. On a road trip this summer I blew a tire and replaced the two rear tires with slightly narrower and higher ones than the front wheels. The technician that discovered the clunking suggested this is what caused this problem.

I recently discovered a "clanking" when driving over bumps. Maybe something is severely loose and rusted free down under... but it was up on a hoist not too long ago and nothing was mentioned to me...

I also discovered that this rumble goes away if I switch to 4WD... so for the past few weeks I've been burning up my drivetrain driving it in 4WD on the highway. I want to have some idea of what could be going wrong and if the truck is a goner before I bring it in for a mechanical fishing expedition .

 

ever find the problem? A little help, maybe? Similar problem. My recently acquired '00 2x4 4runner v6 auto with 240k miles has developed a new vibration issue. At low speed, under 30, it almost feels like a dead miss, smooths out between 30 and 55 or so, then comes back worse between 55 and up, as accelerating. But anytime its vibrating, if i let off the gas it stops. And its in the seat, not the steering wheel (so no front end suspension/tire prob, I think) and not in the brake pedal (so no brake probs either). No clunk when shifting into drive from park, but a bit of one when shifting from drive to reverse. The check engine light has been on for years (evap cannister, yeah, I know, on my list to get fixed, lol), but when this is happening the ck engine light starts flashing on and off. I helped maintain the vehicle for the last 10 yrs, as in offered advice when probs cropped up, occassionally drove truck to help diagnose probs, and took to shop for probs when requested. I don't know that the rear end lube level was ever checked (at least I never specifically told the shop to do it, may have been ck'd as some regular inspection). Getting codes pulled this afternoon. Looking for options, opinions and best guesses. I'm betting drive shaft or rear end.

 

Since no update from MA, will offer mine (update would seem to "complete the circle" and help others resolve issues, so why are there so few?). I found a worn rear universal joint, replaced, and changed rear end lube. Pulled codes, P0171, P0300, P0301, P0303, P0304. Regarding P0171, several possibilities, so started with replacing fuel filter, found water in gas, replaced with new filter, filled tak at rarely used Sunoco station and dumped in a can of dry gas. Back to house and pulled MAF in response to P0171 code; very dirty, MAF cleaner took care of it. I reviewed several MAF cleaner videos, chose to move component on wheel well held by 3 bolts, then remove MAF by removing 2 phillips screws, due to age of air cleaner hoses and possible brittleness. Cleared codes and went for a 15 minute test drive. Much better, but still not perfect, ECL came back one, slight acelleration vibration, rechecked code, P0301, #1 cylinder miss. I figure its the plug, the wire, or the coil, will wsap each in turn and see if miss migrates. More to follow, hope this helps.

 

Oops, CEL ck eng lite, not ECL, sorry, lack of proofing and in a hurry. Also, air intake hose (MAF in section that attaches to air filter box), not air cleaner hose, brain fart from old fart, lol.