Cannot figure this one out!

Chevy Trailblazer
2003
4.2
105,000 miles

About 6 months ago I checked my differential fluids out.
I had a high rev noise when I start out (like it was in low gear going 20mph) but it was immediate from the start. The sound would always be there unless going uphill (then it was fine). At some point on the incline the noise would disappear.
Turned out I have a small leak in the front differential. I filled it with about 2 pints or so and the noise was gone.
Well, the noise is back today. I have checked all the fluids (front d, rear d and transfer case, all full. Transmission also full. I can't figure out what else to check.
4 wheel low will not engage. That has been an off and on problem for a while. The tac is broken so I can't look at that, but it sounds like its high even though it shifts fine. Any advice?

 

I can try to help.

If it's an auto,(I'm sure you know) but make sure it's in neutral and apply the brake, then put it in 4-low. Since it doesn't feel like it's engaging, throw it in reverse after you put it in 4-low. Travel about 10 feet or so in reverse and that might help it engage. It's the same idea to disengage 4WD. Try throwing it in 4WD, hi or lo, throw it in reverse for 10 feet, then put it back in 2WD. Then reverse again to make sure it fully disengages. I have no idea if this will help you, but this is what I gotta do in my truck. Hope it helps.

 

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