Not tryin to jack a thread either but when I got my tranny back from my builder & installed it, it didn't shift into 4th. At all, no matter if I sped up to 90 or just cruised it at 60. This was the 3rd time my builder had built it & he'd already added the shift kit from Superior the 1st time & I never experienced any lagging in 1st & it always bit hard in 2nd & was nice & firm in 3rd & 4th.
This last time however, it wouldn't shift. After a latenight call on that Friday night, he came out Sat. AM & brought all replacement solenoids, harness, force motor, case/valvebody gaskets, 1/2 accumulator piston, 3/4 accumulator piston & various other odds & ends. I had the truck on stands & drained when he showed. We pulled the valvebody & seperator plate & cleaned them up. While I was doing some cleaning, he tested all the solenoids, force motor & the harness. He then checked through the V-body & began to compare the plate/gasket/V-body to each other when he noticed on the gasket between the plate & v-body, there wasn't a provision for a 32nd in. hole in one of the passages.
He'd swapped all the solenoids for good measure & installed new gaskets that actually had the provision for that hole & we put it all back together. It ran just as good as ever & shifted into 4th like a rocket. With Allen stating to check the gaskets is what prompted me to go on this rant. I couldn't actually tell you guys if all the other stuff we did to mine was relavent but I can't help but think that hole being blocked by that gasket had something, if not everything, to do with it not shifting. My trans guy couldn't confirm, nor deny that fact either, ha.
I'm jus sayin.... it never hurts to check everything. Good luck with your search & remedy man. I'm sure you guys will find it. Hopefully it will be something simple.