I was going to mention that most SRAM rear shifters are not compatible with Shimano rear derailleurs. However, there are a few SRAM shifters that are Shimano-compatible and it looks like the TRX is one of those.

The original grip-shifters, SRAM TRX, and Shimano shifters should all be interchangable.

I would double-check that you have installed the new shifter and adjusted the rear derailleur correctly.

What do you mean by "stcks in 1st & 2nd"?