I have. slow drip of transmission fluid. my local mechanic diagnosed the problem as the output shaft seal...he described it as the seal on the shaft between the trans and transfer case. I am not very knowledgable of these things, so just reporting what he said. he told me his is unable to find the correct part ( seal) at Napa etc. that does not make sense to me. these trans were in the H3, Colorado, etc. should be an easy part to source. when I google online I find all kinds of seals readily available. does anyone know what part # and location online I can buy, so I can order it and bring to the mechanic .

 

Find a new mechanic. I believe he/she is telling you they do not want the job.
AS you stated, there are multiple 4L60E rear output shaft seals available with a simple internet search.
Even brand new from GM.

GM Genuine Parts Output Shaft Seal

GM Part # 08677268
ACDelco Part # 8677268

 

Ditto. Its a pretty straight forward job and a mechanic should be able to do it lol!

 

Doubtful the bottle cap seal is the source of the leak. Here's a 5pack cheap on ebay >>>>> 700R4 4L60E 4L65E Input-Output Shaft Splined Seals Viton 5pc
While your in there replace the gasket & O-ring seal.
98-ON GM 4x4 Transfer Case Seal O'Ring & Adapter Gasket 4L60E 4L65E Transmission

 

my mechanic was in there once already and said he replaced "a seal" but it was not the right one. said that I needed the output shaft seal. he didn't charge me for the first job because it didn't work. I live in a very small town where you don't have a lot of choices for mechanics. I suspect he is trying his best...

 

Seal on the transmission output shaft or the seal on the adapter (for the Tcase) for the output shaft?