need your help please

[franky69c10]

Ok I have a 87 tbi Silverado 2wd with a 700r4 ok the other day coming home from work my truck started slipping like it was in neutral I got home check the trans fluid and it was ok so parked it then an hour later I try to drive it but it wouldn't shift out of first gear after awhile it just revs without moving but if I tap the gas it goes but won't shift out of first, so I came come and adjusted the tv cable but that didn't help and this is the thing if I floor it like taking off fast it will shift all gears but only in high rpms but when I take off slow to drive at regular speeds it wont shift out of first...... can anyone help me out