2006tacoma rough shift 4th gear

When speeding up softly at about 45 mph the automatic transmission shifts very very hard, but under heavy throttle it seems OK

The problem is lessened if the gear selector is left in 4th rather than drive, however when fluid is warmed up, throttle off, going down hill it seems to slip or chatter in 4th.

Less than 40000 miles 5000 of which were pulling a light(1000 lbs) covered trailer

 

Hard shifts most likely because of high line pressure in transmission. Could be caused by solenoid in transmission. Have you checked for trouble codes with scanner. Some scanners will allow you to see pressure control solenoid function.