'96 E250 lazy upshifts

I just got a 96 E250 with 185,000 on the clock and am curious as to why this thing doesn't want to upshift (seems like every gear) unless you let up on the throttle. My first guess would have been a vacuum modulator but as you might have guessed it is a E40D and there isn't one. Apparently the previous owner says this just started and he just wanted to unload it. When it is in gear it pulls great, no slippage whatsoever and no other problems. I've read in my searches where the speed sensor could be a problem or the solenoid pack. I don't mind taking the sensor out and cleaning it but don't really want to start buying $200 parts cause they may be the problem. Anyone have any ideas or previous experiences they would be willing to relate? Any help will be greatly appreciated as I don't want to drive this thing til I get this straightened out, Thanks

Have the diagnostics ran on it. If it is the soleniod pack, or some other electronic problem, there should be a code stored for it if the battery hasnt been disconnected. Change the fluid and filter, if you havent already. Look for debris and metal shavings in the fluid and pan. A small amount of fine metallic particles and sludge is normal with that many miles. Big chunks and pieces arent. Make sure it isnt overfull. A little wont hurt, but very much, and the fluid can contact rotating parts and foam up . The air bubbles from this can cause shifting problems. Good luck.
Later
Ed

Well I hooked it up yesterday and had 5 engine codes and no others. Pulled the sensor out of the rear to clean and it wasn't dirty. Changed the fluid and filter, added a bottle of Lucas and topped it off took it for a drive and still acting the same way. I have another tranny here for it but I really don't want to go through all of that if there is an easier fix. It has to be in the valve body I'm guessing. It is now dropping out of drive sometimes when I come to a stop and goes back in when the gas pedal is depressed slightly like it is losing pressure somewhere.Any ideas?

If its dropping out of drive, sounds like the forward clutch is disengaging, and maybe some of the other clutch packs too. Like you said its most likely a pressure related problem. It could be caused bt a number of things, weak pump, clutch piston seals leaking, valve body, solenoids.............If theres a trans shop there that you trust, let them run a pressure test on it. They can usually pinpoint problems from this. If you want to tackle it yourself, private message me with your email address, and ill send you some info on the test procedures. But if you dont have the gauges ect... , by the time you buy/rig it up, its usually cheaper to have a shop do it. Good luck.
Later
Ed