I while back i had an issue that under load, going up a canyon i'd experience what seemed to be "slipping" from the rear... at the time i had bad tires, one side was almost bald, while the other side was maybe 3/4 tread left.... Bad i know, but it felt as though the bad tire would just lose grip and the 4Runner would "surge" a little bit... i just thought it was the tires. it went away when i took care of the tires and since i've put on the 35's I never noticed it but the other day it happend again, not as pronounced, but it was still there....

what could it be? tranny? timing belt? (time for a new one)

 

Anybody?

 

How does your transmission fluid look? Smell burned?

 

One more thing, do you have a transmission cooler? If not, those 35's could be overheating your tranny.

 

Kind of sounds like your transmission- do you get any tire chirp when it happens, or do you just hear the engine shoot up in RPM's. I'd get a tranny cooler for sure, cheap insurance.

 

id say tranny

add a cooler, flush it out, put in synthetic, new filter