Wow, 2 bad transmissions? I'm hearing in my head, "The only common denominator in all your bad relationships is you."
Could be the Texas heat causing the premature failure.
My '01 Burb is now owned by a buddy, has 270k miles, rusting out, but original transmission. I abused that thing so bad that it's wearing rear axle number 4, but still the original, allegedly weak, transmission.
And my '08 is closing in on 200k miles, also beat the snot out of it, have had the fluid as high as 240°, and still the original transmission.
I question the benefit of intervals shorter than 50k miles. I do 50k - trans, axles, t-case. If you're changing more frequently, I'd be curious if it's really necessary, i.e. are you checking the color/smell of the fluid? Mine still looks pretty good at 50k, and again, I'm almost always towing heavy with mine.