Assume you checked the obvious like the fluid level? Not sure the process on a 4Runner for checking the level but I assume it is similar to an 80 where the AT needs to be operating temp and then with the engine running, check the level while parked on a flat surface.
Do you know the service history on the ATF like when it was last changed? If not, I'd swap it with fresh so you'll know. Does the 4Runner have a ATF cooler? If so are the fins etc. in good shape so there is good airflow?
After I swapped the AT pan on my LX I was about 2 qts lower than I needed despite dumping 4+ qts in. It took a week for the symtons to show up with tranny slipping. Any slow leaks that you have that are causing the fluid to gradually get low?
Jeff Z. (the "not quite as skinny" one)
'97 FZJ80 Antique Sage AE #267, stock
12/74 FJ40, 2F, SM420, 4" Lift, ARBs, 33" MTRs
"...anything else i can do for you guys, how about i wash your car or mow your lawn while you figure out your firewall system? I am now boarderline insane/unibomber." Kipper
"That assumes I'm even capable of pulling and stabbing..." Jacket
"I really like having a detachable unit." Beater