91 W150 3.9L auto no power!

[Dcdoc45]

I got this truck several years ago on trade. PO let the engine oil get low and had a rod knocking so I purchased a used 91 3.9L V6 that came out of a dodge Dakota. Only thing I had to change was the exhaust manifolds.
With the first engine in it no problems accelerating going up hills, etc.
While I had the engine out I took the tranny and Tcase to a mechanic to rebuild. The truck starts and idles fine, revs up in park and neutral driving down a flat level road it shifts from first to overdrive fine. If I'm stopped on any incline, or if I try to go up a steep hill the truck barely goes. I can manual shift to 1st, no difference. I shifted down to 4L just to check, had a little better throttle response but not much. I have followed the shop manual procedure for adjusting the TV cable but that didn't help, it actually got worse, before I adjusted the TV cable I could at least stomp it to the floor and it would drop a gear, now it won't if I stomp it, but it will on a flat road with no incline. I can't figure out what the issue is. Engine does not smoke at all and idles fine. Any help would be appreciated, about to take it to a mechanic, I've gotten this far and this has me stumped.
Thanks

 

[Dcdoc45]

Update:
used a code reader and pulled these codes.

37
51
21
37 said "torque converter clutch (part throttle solenoid) or shift solenoid C, Aijin transmission or transmission relay circuit in JTEC RE transmissions.

The emission status on the reader was red if that makes any diff.

I looked on the driver side of engine bay and there are 3 different relays, I can hear one buzzing for a second after I turn off the truck intermittently.
Which one is the part throttle relay? does anyone have a picture one installed thanks.