I'm Sorry to say it but I have to agree with the above comment. It sounds like you need a rebuild. To prolong this, shift through first and second until the tranny warms up them try putting it into drive. This will last you a while, but the problem is still there. your looking from 1500 to 2500 dollars to repair.
I had the same problem and theres not much you can do about it.
Possilbe causes; the engine ran hot (rad problems), fluid is done (do tranny services about 3-4 times a year, VERY IMPORTANT).
My suggestion is to find a rebuilt one around your area, buy it, have it installed, and, if your mechanically aware, rebuild your old one so you have a spare. reason being these trucks have terrible transmissions on years 91-94 and you better get used to this if you plan on keeping it!
Preventaive maintenance;
(1) Do a tranny serviceing about 3-4 times a year
(2) Have your rad serviced 2-3 times a year depending on your climate
(3) Put in a transmission additive on every servicing
Good luck
Myles
94 XLT
P.S. Pay close attention to your rad fluid!!!