OK i have a d250 automatic trans 2wd I can not pull anythig i put a 14ft enclosed trailer behind it and could not get overe 50 and 40 on the hills with no trailer i feel like I am driving my old peter built in 2nd gear I have ran the over head changed fuel filter staight exhaust cold air intake and still nothing any ideas being a diesel mechanic i am thinking it has something to do with the trany

 

What is the fuel pressure and boost pressure doing.

 

how many miles are on the transmission?

 

I too have a 93 w250 with the same identical problem. I noticed in the beginning my fuel mileage dropped off considerably and found the torque converter was bad so I decided to by a new transmission from ATS. After I installed the new trans I got the fuel mileage back but the truck makes absolutely no power. I pull a 24' enclosed trailer and the truck pulls like a slug. At wide open throttle it will barely will run 55mph. Ive owned the truck since new and the truck is bone stock with 153,000 miles.

 

Guess I shouldn't be upset with 90 mph with my 44ft enclosed on level ground .
Check and see if your torque convertor is bad. Do a Stall test if stall speed is high it indicates transmission slippage. if it is low on a properly tuned engine the torque convertor overrunning clutch has a problem stall with factory convertor is 1700-2000 rpm? that is out of my 92 factory service manual. hope it helps a little