4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

[Second Series]

4x4 engages in 2wd at start. 1990 k1500 5-speed manual.
The 4x4 light comes on right after I start the truck, stays on for a couple minutes. The front axle seems to be engaged because I got it up to 50mph and the truck was shaking. I was going to turn around and go home, but the light went off so I went onto the freeway and did 70mph smooth. This is with the floor shifter in the 2WD position.
I am able to shift into and out of 4WD with the light turning on/off with the typical delay. I’m looking at the light by the floor shifter.
I just got the truck back from the shop and trying to troubleshoot, if I can fix it myself that saves me from taking it back to the shop. The work done: Clutch, so the transmission and transfer case were removed and re-installed. The front Diff was leaky so that was R&R’d also. Since both ends of this system were worked on I don’t know to look at one or the other.
Last night I started to look at it. I disconnected the actuator, and put a test light on the connector. Start the truck in 2WD. A moment later the test light came on, and stayed on for a minute or two and then turned off.
Is there a relay between the T-case switch and actuator? I think the next thing I’ll look at is unplug the T-case switch and check the switch with an ohm-meter to see if it is changing when I start the truck. Does anyone have any idea what could be causing this?