Toyota Repair: 90 4-runner trans, transmission failure, first signs

Question
Trans has to be hot before it shifts into lockup then overdrive.It shifts when temp gauge is nonmal.What do I look for?  Thanks.

Answer
I have seen this happen on older 4-Runners including my own which has bee doing this for a while, it seems to be the first signs of transmission failure, a used transmission is the cheapest way to go,(about $1,500-1,800) I have not replaced mine, I don't drive it much anymore and will probably sell it to a wholesaler soon.