downshifts when I let off the gas at 2000 rpm

Hi everyone, I'm new to this community..I have an issue I think? I have 2 tacoma's (a 2000 2wd and a 1996 4wd both auto transmissions)...
I just got the 96 two wks ago. since i got it when driving on the freeway 55 or 60 mph and at 2000, 2100 rpm's I take my foot off the gas and it downshifts every single time (not intermittent), and soon as I give it gas again it upshifts (it seems as though it should be the opposite right?)..I have replaced both the Throttle Position Sensor & the Transmission Speed Sensor
as well both in the last couple days no change! my 2000 2wd tacoma does not do that ever.....is it time to take it to the tranny shop? I have no other issues with the transmission. no noises, no hard shifting, doesn't pop in and out of gear, fluid is full and doesn't smell burn't, no leaks underneath...my shifter is loose as all hell though......think I covered everything....it's a 4x4 with 221,000 miles (runs good otherwise)....any ideas would be greatly appreciated....thx

UPDATE: tranny shop just said it's either the TCC Solenoid or the Torque Converter or both....by the way I never had a code thrown, which if the solenoid goes bad doesn't it automatically throw a trouble code?