[B-300]

It's not likely the 1st/rev band since it isn't engaged unless the gear selector is in 1st or rev.
Slipping could be several different things.
1. The rear clutch, also called the forward clutch could be slipping... Not to likely since it works at speed. (This clutch is engaged in all forward gears)
2. The overunning clutch (sprag) is worn and starting to slip... Try taking off in 1st, this engages the band and will prevent slippage. (Only works for 1st gear)
3. Your torque convertor sprag is slipping, causing low torque multiplication... A balloned convertor will do this also. (A convertor stall speed test would tell.. It would be higher than normal rpm)

 

[B-300]

Confusing?
With the selector in Drive the power in the planetary gears is held by the overunning clutch... It works in parallel with the 1st/rev band.
With the selector in 1st the power in the planetary gears is held by the overunning clutch and the 1st/rev band... (This is why if you pull the selector into 1st at speed and it downshifts to 1st you can break the rear wheels free do to engine braking.).. I wouldn't recommend doing this for safety reasons and because it puts a shock to the drivetrain.