Toyota Repair: 4Runner tranny, coolant temp, shift into overdrive

Question
I have a 93' V6 3.0L 4 Runner.  I have read similar problems on the net and can't decide where I should start.  The tranny wont shift into overdrive or really 4th gear until the engine reaches temp, which is about 7-10 min of driving.  I know this is a normal operating condition, the temp sending unit has to send the command to shift once the engine is at temp, but it started happening in Nov and worked fine before then.  I have not replaced anything, the engine seems to warms up fine so I am hesitant to replace the thermostat.  Not sure where to start or which parts to buy or even how to check the parts that i should replace to make sure they are bad.  I replaced the tranny fluid in Nov but that is the extent to the repair i have done with the transmission.  Any advice is much appreciated.  

Answer
It is indead normal. The coolant temp needs to be 158 degrees to shift to O/D
If its taking longer, a t-stat would be a good place to stert.
Then maybe the coolant switch for trans, located in the coolant passage on the intake ceter of motor at the firewall.

Mike