Problem with 700R4 Tranny

I've got an 83 S10 2 wheel drive with the 700R4. The tranny won't upshift unless I over rev the engine and back off the gas. The TV cable has been replaced, and, the filter and fluid have been changed. There was no debris in the pan when I pulled it to indicate any damage to the tranny has occured. The tranny has about 50K miles on it. Any experts out there on this tranny who can advise me on a fix??

I'm not wanting to give you the bad news but I am quite afraid to say my experience with the 700R4 transmission in that case has typically ended up with a bad band or two in the transmission.
I had a bad run with those trannies a while ago.
I do have a transmission sitting that should be good if you would need it,
I nolonger need it as my S10's all are now 4x4 with the standard tranny.

I'm not sure the exact trannytype I have. But it acts up like that when fluid is low. Topping it up fixs. Maybe an additive to help shifting ? If it uses vacuum lines, be sure no vac leaks anywhere. Or a shop inspection at low cost to just remove pan may be able to adjust bands ?