I did a search already and have found some things about DTC P1870

But mine is different then the treads posted.

Truck:
1996 S10 Blazer 4 Door
4.3 vin W
4L60E (replaced alittle over yr ago with a rebuilt) Has 65k on it currently

Was pulling a really light load (under 500 lbs) back home when the truck acted like a GM car I had, just kept kind of jerking with heavy vibration at idle, and sometimes it would just stall out at idle or take off. Sluggish up shifts and acted like it was loosing power. Once you got over 20 mph if you let off the gas the vibrations would stop, but as soon as you started to go again it would return to a shudder/vibration. Was unable to get it over 55 mph to find if it was and OD issue or something else. And the MPG went out the tail pipe harshly (1/4 tank used in under 50 miles)

What I found on my GM car was that the TCC had gone bad, but in all my reading of the DTC on the Blazers, it seems that there are many things that could do this that are not even related to the transmission.

DTC when first pulled:
P0305 (cyl 5 misfire) I have had this one several times normally just clearing the code fixes
P1870 Transmission Slip/issue (common general code)

Codes after cleared but still acting up
P0300 (random multi misfire)

I have been diggin into it, have found a few things that I am correcting on the motor end.
Cap
Rotor
Wires
Plugs
FPR (as they are prone to letting loose without much warning so figured while I had it open, would just replace it now instead of waiting for it to fail)
Clean EGR with new gasket (this one with screen)
Checking and replacing all vac lines (if they look rough replacing not taking a chance)

But just incase this does not fix the issue, I wanted to post here and see what advice or helpful suggestions I can find on the P1870. I have already looked into the valve body kits, and I do have an extra valve body and some of the solenoids (from a core transmission I had planed on building a stout replacement) sitting incase I need them. The Core transmission had 3rd and 4th out, simple band replacement, but figured I would go and build up a nice stout transmission to swap out this spring. As I do a fair amount of light load (under 3,000 lbs) pulling.

Tansmission fluid is still red, doesn't smell burnt but is alittle discolored, going to replace the filter and fluid so that I can check the pan for junk. Its about due for a new filter anyway, just wish it had a drain plug. B&M makes a nice trany pan with plug but that is costly just for a stamped pan. They charge like its bullet alum. Going to check the Torque Converter for discoloration.

Thanks in advance

 

You really need to work through the diagnostic procedure for the P1870 code to determine the root cause of the problem. If you do not have the equipment to do so, you'll likely have to take it someplace or you may end up needlessly replacing components.

 

I almost have the engine back together, just have plugs left to go. Once I am done with that I will see if it is still jerky and rough idle like before. If so I would have to lean towards a bad TCC solenoid which would mean dropping the transmission.

It functions in all gears (forward and reverse) just dont know if its hitting OD as I could not get it up to speed to find out. With the feeling of the TC locking and almost stalling the engine at idle, I didn't want to push it and do damage to the transmission that would call for a R&R as I kind of need the truck tomorrow for a road trip (no pulling).

 

All back and running, cannot seem to get the P1870 to return, but getting an anoying P0300 that just wont go away. Even after Cap / Rotor / Plugs ... Ohm out all the wires and they are almost like new. Few times said cyl #4 this time instead of #5.

Removing battery cables for 10 min (or so) to clear things out and then will see how it runs.