First off, describe the slippage. Is it only from a stop, or in certain gears, or all gears and scenarios? WOT, 1/2 throttle, 1/4 throttle? What about at a steady highway speed on level ground? Autozone will pull codes for free... get scanned and post if any.
The main culprits for a true slip in these transmissions under part throttle are the plastic accumulator pistons and the PWM circuit for the torque converter clutch. The last one normally isn't a factor in the trucks since the TCC lockup is usually firm and direct and it doesn't drag the TCC at low speeds like GM programmed many of the cars to do.
If it slips in all gears then the forward accumulator piston is likely cracked and causing the forward clutch to slip (don't drive it like this! FWD clutch is a bad one to fry as it can damage the drum). In 1st gear it's unlikely that the lo/roller clutch is slipping as this is pretty strong considering the gearing advantage. 2nd with be the band, 3rd the direct clutch, gears 1-3 all use the forward sprag, and 4th gear uses the forward clutch, band, and direct clutch. In other words, if the slip only happens in gears 1-3 then the sprag is likely gone... manually shift it through 1, 2, and 3 on the column shifter and see if the slippage is still present.
