94 2500 5.7 4l60e. R, 1st, and 2nd work independently if I select them, but in D it wont shift at all. Around 25mph it feels like the power from the engine to transmission just goes away. I can keep throttling but no response until I slow back down under 20. Google says maybe due to old fluid and/or stuck solenoid. I have an appointment to drop it off at the transmission shop next week, but is there a change I could get it going with a fluid and filter change?

Why not do a fluid/filter and see what happens.

I don't think a fluid/filter change will fix this. It'* worth a try for fairly cheap. Usually an early sign of filter plugged up etc. is delay engaging gears when you select them, not engage two of them fine then don't engage 3rd and 4th. And . . . if you change fluid and filter then the transmission shop will have a harder time with diagnosis without the current fluid and filter in it.

From this part of your description:

. . . I believe you are saying that with "D" selected is will shift 1-2 fine, then when it attempts to shift to 3 it doesn't engage 3 while it thinks it has done so.

For 3rd gear, both solenoids need to be off. First gear needs them both on and second needs one off. Based on this I believe it has a bigger issue than a shift solenoid.

You say this is a 2500 with a 4L60, does it have 8 lug or 6 lug wheels? I know they had a 6-lug 2500 in or near this era. If it is an 8-lug truck, then are you sure it is a 4L60 and not a 4L80?

How many miles on this truck?

In "D" is doesnt shift at all, just stays in 1st. I can select 2nd on the shifter and drive it the same way. It'* the LD 2500, 6lug rear

A possibility could be a restricted fuel filter limiting fuel psi and thus a loss in power and that could cause the trans not to upshift on it'* own. Have seen this many times. Just a thought.

Ah okay. I still don't think a fluid/filter change will fix this. It might be worth a try for fairly cheap, but i wouldn't do it. If it goes into 1st with "D" selected, that tells me that both solenoids are working. I'd have a quality transmission shop look at it next.

How many miles on this truck?