WHY Solid "CLUNK" when shifting from OverDrive to Standard in E4OD Tranny?
 

OK Guys, I've read the posts about tranny shudders under various conditions, and the fact that changing fluids in both tranny and torque converter fixed the problem for many, but it hasn't for me. Just bought '92 F150 XLT with E4OD. When in OD, whether it downshifts automatically (or by my use of selector) into standard gearing I get a very noticeable "clunk" or "shudder". The truck has been doing this for several months, and I just had the fluid changed, which made essentially no difference. In standard gear, I get smooth, tight shifting. Is the OD unit going bad? What's up?
TIA

------------------
'92 F-150 XLT, 4x4, 5.0L Supercab, Long Bed, Cowpie eatin' Mudders on Chrome spoked wheels.