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Might have a look at this:
- http://www.4crawler.com/4x4/ForSale/Shims.shtml#FAQ1


I saw that while I was doing research, but dismissed it since this vibration came on rather quickly without any changes to the truck. Had I lifted it, or had previously disassembled something, I could see either driveline angles or phasing being possible causes, but it has been fine for 20+ years and nothing has been apart. I did make sure I marked everything before taking the driveshaft apart, so the phasing didn't change.

'89 Toy 4x4 Xtra Cab SR5 V6 266k miles
'01 Tacoma 4x4 Xtra Cab 2.7L
'97 Toy 4Runner Limited