95 F350 dually trans problems?

[techno550]

Trying to pinpoint the problem with my trans and have a few questions.
The trans slipped/shuddered a little at light throttle in 1st and 2nd. Changed the fluid and it got a little better but not gone. No shudder or any problems off throttle. No problems at all at high throttle, just grabs and goes. No shudder, no slip, no nothing.

So now I was thinking maybe clutches or TC?
When is the torque converter locked? Is it locked at high throttle in all gears? If so maybe I have a bad torque converter. Or is it clutch slip instead, but not due to bad clutches (I assume it would still slip, or slip worse at higher load), but instead due to a bad solenoid (or pressure issue)?

Any ideas?

Thanks