2000 Tahoe Z71 w/ 5.7 tranny trouble

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My 2000 Tahoe 4wd 5.7 with the 4L60E tranny is not shifting out of 2nd. Feels like it is in neutral after highway driving. It is throwing a P1870 code. We had trouble with it last month and I dropped the pan, changed the filter and the Shift 1-2 and 2-3 solenoids, filtered the oil and replaced it. It drove beautifully for a week! Yesterday it drove well for about 30 minutes, then upon exiting the fwy, it felt like it was in neutral. I am hoping it is a relatively easy fix as I can't manage a transmission rebuild or replace right now! I have researched it and found that it MAY be the TCC valve. The fluid was dark brown when we changed the filter, were advised by a tranny shop to not put in new fluid, due to the 205,000 miles on it and no idea of when it was last changed. How hard would it be to change the transmission valve body?

When I pulled the dip stick to check the tranny fluid level last night, it smelled different than it did last week. Not sure it is a burned smell, but definitely has an odor now.