Here is a recent related service bulletin:

Harsh or Delayed Forward or Reverse Engagement:
This could be caused by a sticking line pressure control (PC-A) solenoid. Typically the line pressure will be fluctuatiing wildly (up to 500 PSI!). To service, first remove and insdpect the external cooler line filter. If it contains fine gray metalic debris. If metal is found, the source is most likely a defective revese planetary pinion shaft that is coming out of the carrier and contacting the case or adjacent components. The transmission will need to be disassembled and repaired as needed or replaced. Broadcast Message 3250
If little or no debris is found replace the line pressure control solenoid with P/N 4C3Z-7G383-AA. Broadcast Message 1586/SSM 17874/18051
This repair will require removing the solenoid harness, so a new harness sealing o-ring (P/N 3C3Z-7Z276-AA) will need to be installed. In some cases the solenoid body will need to be removed from the trans to remove the harness and solenoid. In this case the solenoid body gasket (basic P/N 7C155, trans tag number needed) will be needed as well. Note: Do not loosen the 10mm head bolts on the solenoid body. Only loosen the 8mm head bolts.
If the solenoid has been replaced and the condition it still pressent, suspect a contamination issue of the solenoid or a sticking pressure relief valve.

 

SBV,

Thanks for the great info and help you provide here!

I have been wondering what that slow speed clunk was, when starting out after changing direction.

Now I don't have to worry about it any more.

 

Welcome, but this is what FTE is all about. I just happen to be driving one of these a little longer, read alot and share back and forth a lot of info with the other ol'timers on this forum (and one youngster who has more first hand experience with these trucks than most of us will ever have). Thanks to the moderators and webmaster that make it possible.