I'm sure y'all are tired of me by now, but, I at least think I've made some progress. Tried pulling the tv adjustment sleeve all the way out to see if that made a difference in shifting, it did. Shifted super early rather than super late. Then I had the thought, that if the gas pedal has no effect on the tv adjustment as it should, perhaps there's an issue with the throttle linkage to the gas pedal. Then I thought, what if I set the tv cable all the way back to the firewall, then pull back all the way on the throttle linkage arm (instead of the pedal) and see if that makes a difference, and it did! The TV sleeve popped back out just a little bit, which brought the 1-2 shift down to 20mph. So I adjusted the TV sleeve out a little more, and that finally brought the 1-2 shift back to 15mph, 2-3 at 25mph, and 3-4 at 50mph. Down shifts seem to be okay too, didn't try out 4-3 down shift, didn't want to push my luck. At this point I'm thinking I may have the same situation Convert2diesel had. Just glad it's shifting like it should now. Throughout the testing, temps stayed below 180.
1988 K10 Suburban. 6.5L NA Goodwrench Engine: 506 block cast on August 12, 1996. Heavy Duty built 700R4. Full-floating 14 bolt rear axle. 3/4ton 10 bolt front. Brand new 305/75R16 Procomp AT Sport tires on brand new 16x8 steel wheels.