4l60e doesnt work past 20 mph
 

I bought a 2000 Blazer ZR2 4x4 with a 4l60e which needed a new trans filter. I replaced the trans filter in it and changed the fluid, which was black, and tried to drive it. It would drive fine in 1st, shift into 2nd and the just keep revving and never shift into 3rd. I let It sit for about 3 months, starting it up and puttering around the yard every now and then, while tried to research the problem to no avail. But now it doesn't seem to work past 20 mph at all. I put the selector on D and it drives to about 10 mph then acts like it's in neutral. If i put in 1 or 2 it will drive but only up to about 20 mph then it acts like it's in neutral. 3 sometimes works, sometimes it doesn't. I am at a loss as to what might be the problem. Anybody have any ideas?

Obviously one or more of the clutch packs are burnt up. Needs overhaul or replacement - not a simple fix.

If you can get to upshift by very gradual accelerations but not when you gun it, that would be consistent with bad clutch packs. But no matter what the fault, I'd guess you are likely looking at a rebuild or a replacement.