So took the truck for another test drive and she is not changing to 4th OD and it doesn't appear that the converter is locking either. Checked the oil again with trans hot and had to remove a further 1/2 litre of ATF, now it is reading the correct level. When driving she wants to change but doesn't quite make it you ge the feeling of her catching the gear but dropping again. Looking at the Transmission Trouble Shooting TM (page 43) it looks like there could be an issue with the shift solenoid A or B, loss of supply voltage, pressure switch fault etc. It appears that the issue has started when I changed the batteries as the old batteries died in the cold weather. Replaced with military batteries for the HMMWV but this seams to be the start of it all. No issus with trans except change down when I hit bumps in the road. Might have been coincidental. Oh and I found that the trans warni g light had been unplugged, so trans light is on. Need to check for codes now.