OH NO! Is my TH400 about to bite the dust???

[Chevy3+3]

First of all my truck is an 88' Crew dually 4x4 with 134K. My tranny fluid is at the correct level.

About 3 weeks ago I noticed that 1st gear felt like it was slipping some when pulling a heavier load. It would eventually shift and 2nd would pull fine and 3rd would as well. The last few times I drove it I noticed that now I'm getting slippage in 1st gear without a load. My 2nd & 3rd gear are still great.

Should I be prepping myself for a tranny rebuild? Or could it be something else?

I just wasn't sure what the warning signs were for a TH400.

On to my next question......Simplicity would be to go back with a TH400 but overdrive is something that would be nice. What would be the easiest overdrive tranny to swap in and would mate to my np205 tcase. I know a 4l80E would be an option but I would need a stand alone controller I believe. Im not sure how to do any of that.

I've also considered rebuilding the TH400 and putting a gear vendor on instead of an overdrive tranny. Just not sure what to do

I would also consider swapping in a nv4500. I think I have access to all the clutch components in a junk truck.