tranny vs thermastat

my 2001 Explorer has 103,000 mile on it,recently it has been over heating climbing hills,not on flat surfaces.It appears that the RPM's are higher than normal also, and when they go down the temp does also. Is it the thermastat sticking or the transmission going?

Do you have an automatic or standard transmission. How much are the rpms going up? Are you pulling a trailer this also causes problems. You might have a radiator gradually plugging up and a transmission going bad. More information is needed.

automatic transmission,not pulling anything, it started about a month ago very intermittant just climbing up hills, within the last day or two,it is occuring more often. The RPM's go to about 2.5,while going about 25-30 mph

bought a thermostat today and new o-ring, are they hard to replace? or does it envolve draining the radiator?

You need to drain the radiator enough so the water doesn't run out when installing it. If both the top and bottom radiator hose is hot this isn't your problem. If you are in high gear at this RPM the transmission is slipping.
Remove the transmission dip stick and smell it if it has a burnt smell its definately going bad. If its not shifting properly it may be the shift solenoids.
If you know of a good transmission shop ask for there opinion. Use a shop that does a lot of Ford's before you let them work on it. AMCO will pull the transmission codes free.

top radiator hose was hot,bottom,room temp...trans fluid smelled like trans fluid...

That sounds more like a plugged radiator. You need good circulation to cool the anti freeze down. If it was normal temperature and no hoses were collapsed and the water pump belt is tight and running the pulley fine I would check with a garage or buy a new radiator from somebody like Pep Boys who have decent prices on radiators. Try turning on the heater when it is hot it should cool down immediately.