2000 blzr transfer case problem

Here's the problem: dash light is on when in 2 wheel high, it works in that gear. When i push the 4 wheel low button, both 4 hi n 4 lo lights flash and then off and i hear and feel the transfer case shifting into 4 wheel low- lurches forward and "crawls" so i know its working correctly. I push the 4 hi button and i see the lights dim for 3-4 seconds and then they brighten but it won't go into 4 hi. I can then shift into 2 hi and no problem. [this is all done while sitting still with vehicle in neutral] So it boils down that I have 2 hi and 4 lo working but not 4 hi. The lights dimming sounds to me the transfer case motor isnt working correctly to advance the transfer case gears into 4 hi- if thats the way it works. Any suggestions as to where to look next for this problem. BTW- 154k miles on vehicle....Thanks in advance for any info.! {I do have a spare transfer case unit just in case I need to pull the original.}

I had the same problem with my 2000 Blazer with 200, 000 km (120,000 miles). Check the transfer case relay module.
It's located in the right side wall of the passenger side, just ahead of the door.
You'll need to remove the plastic wall cover panel to see it.
Remove the module,(3 nut) & the plug socket & check for corrossion.
The contacts get corroded. (blue oxodation). I found 3 of the bottom were corroded. Cleaning these up solved my problem.

Mike D