2000 Z28, 4L60e, 2.73 rear gear:
Lot of miles, no transmission servicing lately, fluid looks good.

From a stop, not heavy acceleration: engine accelerates to 4000 RPM before shifting, and drops to 2000 RPM, so I think that it is third gear.

How bad it this?

 

what about reverse my buddy had a 2000 s10 with the same trans and his 2nd went along with reverse it was the shineplate (i think thats what it was called this was like 5 years ago)

 

You need to determine if you're experiencing a late 1-2 shift, or if 2nd really is slipping/gone. With the shifter in 3rd to keep the trans from gliding into overdrive, can you feel a 1-2-3 shift? If second wasn't there, the trans would either stay in first, or the engine would rev (free wheel with no forward torque) in second till it hits third. Those two scenarios would be from two different causes. If I were to give a guess, it sounds like it may be your 1-2 servo, as the piston/seal can wear and leak fluid pressure. If that's the case, replace it with a Corvette servo. That's a popular upgrade to give better performing 1-2 shifts.

 

its not a shineplate its a sun shell but thats only if you lost 2nd reverse and OD so youd only have 1st and 3rd... when that went on my buddys truck we didnt notice it until we went to back out of my driveway and the truck just revd other than that the truck seemed to drive fine (he drives like an old lady so we didnt notice it didnt have 2nd)