4l60e downshift issues at 50mph+

[redlinefidelity9]

ive searched about 50 threads and not seen a one that explained all the issues im having so i come here.. 99 silverado 160k 5.3l z71

trans shop wants too much to do a diagnostic because its more than a courtesy... bull

on the highway traveling a steady 50+, i put the pedal down (wot), the trans will downshift and on more than one occation if i dont let down from WOT the thing will not upshift at the top of the shift point! after roughly 3800rpm's it feels like i loose alot of power and "slipping" is occouring. Then just today it did it again and i had what sounded like a backfire through the intake (very violent) and a loud "metal clang" noise when the trans violently upshifted. the slipping at higher rpms has been going on for a couple months now, so ive been easy on it and only pass when necessary.. but this last time has me worried pretty bad.
one day when it first did this slipping at high rpm it did hit rev limit... if i dont WOT, i never have an issue like this~ along with this an irregular upshift occours for several miles. feels like a broken trans mount (bouncy but no metal clanking) and i know its not a broken mount anywhere

i dont know if this is associated or not but my driver side exh. manifold has the typical broken rear stud/bolt - no biggie...but since this last backfiring metal slapping event has happened, at idle i have a small hiss sound coming from that same area. unless it blew the gasket out more than what it was, i fear there is a valve issue as it doesnt sound like any exhaust leak that ive ever heard. if anyone has ever heard a bent or burned valve leak, you know what im talking about.

[Blue Bowtie]

The 3-4 clutch is one of the weaker parts of the transmission. If the vehicle has been used for hauling/towing, it may be slipping. What is the fluid condition? What is the line pressure in D and OD? Under normal loads, does the trans upshift through all ranges normally?