help with trans

[farmboy's toy]

Ok ill start this off right, im new here. Bear with me. My grandpa has a 2004 4.0 v6 4x4 automatic ranger. GREAT TRUCK! anyway, recently it has developed a very confusing issue, It kinda has its own mind on when to shift and to what gear. When you really get on the gas from a standstill it takes a long time to engage second then sometimes shifts back from third to second then back to third. i replaced the trans fluid and filter and they both seemed really clean. The trucks got only 65,000 miles on it. Any ideas on what could be going wrong? i would love to get this thing shifting right cause he has plans to build the engine up.

 

[jrpdash9]

my ranger will do the whole 3rd to 2nd then 3rd again every now and then idk y