Transmission Not Engaging Downshift When Accelerating Hard

2006 Tacoma TRD Sport DBL Cab 4.0 V6

when cruising highway at 55 mph ish and stepping on the gas to accelerate up a hill or to pass someone truck is not engaging what I think is second gear. It feels like it engages for a second but then slips and the RPMs go up to about 4k and there is no power to the wheels. When I let off the gas slightly it engages 3,4 or OD. Ultimately under load if feels like it downshifts, engine revs up but then it seems to instantly pop out of gear or something. No CELs. Thoughts?

 

Low fluid level? Fluid and screen need changing? When is the last time you service the transmission? I'd start there...

Maybe @gearcruncher has some input.

 

A good writeup with lots of info and links for the A750E transmission by @gearcruncher here. Hopefully something there helps you, if not you may have to wait and see what he thinks, he is the transmission guru on here.