I had this same exact problem!
'94 ATX with 63K on the odometer..
The 1st to 2nd shift would feel really hard during partial/light throttle, felt like I had a shift kit...funny thing about my situation, is that it would only shift really harsh from 1st between the rpms of 2500 and 3000 (light accel)to 2nd..and it got worse as the car would heat up..
Luckily I have a Ford Warranty also...and after complaining over and over again, they finally replaced the tranny and 2500$ in work later it has been running pretty good (no more harsh shifts between 1st and 2nd)..
Even tho my tranny doesnt shift rough anymore, It does get a bit sluggish when the car heats up...so since Im in for 60K service coming in 8 days, I asked Performance Plus to replace the VSS just to see if it will help (Since they are dropping the Y-pipe to replace it with a performance one)...
Take care!
Joel
'94 ATX