Auto Transmission Problem, Help?!

I was driving my Tacoma earlier today, and as it shifted into 4th it made a hard "thunk" noise while engaging the gear. After in gear it was fine until it down shifted and did it again. I got home and checked the fluid, and it seems fine. I did a little inspecting and didn't notice anything visibly wrong. I took it for another test drive, and it shifts from 1st-3rd smooth and easily, but once it hits 4th it has the hard clunking shift. It didn't matter if I was hard or easy on the throttle, once it hits 4th it shifts really hard and makes a "thunk" sound. Any ideas? It has me a little worried.

Details: '01 Tacoma DoubleCab V6 4spd A/T, 71K miles

 

whens the last time you dropped the pan and cleaned the filter?? sounds like filter may be stopping up.......or a synchro is going bad.

 

The truck has 71K on it, and I haven't ever dropped the pan. I bought it with 44K 5 years ago, and I'm not sure if it had been cleaned before or not. I don't tow anything with it, so it's had pretty easy highway miles on it.

 

get the old fluid out....drop the pan, and clean the filter screen(brake cleaner works good for it)...and put new fluid in.

towing would be a bit harder on a tranny....but even everyday driving will cause the filter to get dirty and clog up.