My last chance to fix 3rd gear start problem

I am working on a 1998 Blazer with over 200,000 miles on it. I have replaced most of the regular consumer parts with lifetime warranty parts. The motor is strong and runs great. I have a problem with the transmission. I changed the transmission. Twice! I changed, ignition module, coil, cam position sensor, cap, wires, O2 sensors, catalitic converter, ... you get the point. I have a code P0758 Which is shift solenoid B Electrical. I have AllData on this vehicle and it just tells me to check the power going to the transmission. I have gone as far as to make a wiring harness out of an old trans. to check the power actually getting to the solenoid I have power there. But what else I have no idea. Where do I go from here? I don't know what else could be keeping it in third. I need to know where I can find the actual readings for the wiring harness going to the trans. It is the 20 pin connector I think.
If I can't fix this soon, my friend will sell this vehicle and it really is a great car. It has always pulled through. Please help me keep this car on the road. Thank you.

The 2 shift solinoids are inside the transmission. I'm considering having both of mine replaced and my local trans shop said they'd replace both for $300...I'm not good with trans pan gaskets, mine always leak.
I can't remember the shift solenoid brand name but when I talked to them they did mention the brand recommended by an online trans expert on another site.
Oh I remember..Rostra shift solenoids.

The repair flow for that code involves a little more than a voltage check.
Did you read all of the way threw the all data flow chart?
Have you got a good a good transmission capable scanner?
Ant other codes?
Check your PM mail.
Always check wiring and plugins and fuses and power and ground feeds.
If the solenoid does check bad the dealer should have it for around 30-40US$
Copy and paste the pdf and Go threw the complete repair flow chart and post back how it goes.
MT

The 2 shift solinoids are inside the transmission. I'm considering having both of mine replaced and my local trans shop said they'd replace both for $300...I'm not good with trans pan gaskets, mine always leak.
I can't remember the shift solenoid brand name but when I talked to them they did mention the brand recommended by an online trans expert on another site.
Oh I remember..Rostra shift solenoids.

I think you can get the solenoids from a GM dealer for 30 -40 US$
Replacing them with a filter change is a piece of cake.
It is a 10-15 minute job to replace them with the transmission filter off.
But as always check the wiring and other stuff .
Are you getting any codes on yours?
MT

Thanks MT,
My 3-4 failed, burned up a clutch pack & cost $1200 w/torque converter replacement a coupla years ago.
The trans is working fine now.