For a given data table see sample below, I only want to keep Difference column for values greater than 2 by Unique\_ID, Without deleting the NA rows . 

```
My_data_table <- structure(list(Unique_ID = structure(c(1L, 1L, 2L, 2L, 3L,
                   3L, 3L, 4L, 4L, 4L), .Label = c("1AA", "3AA", "5AA", "6AA"),
                   class = "factor"), Distance.km. = c(1, 2.05, 2, 4, 2, 4, 7,
                   8, 9, 10), Difference = c(NA, 1.05, NA, 2, NA, 2, 3, NA, 1, 1)),
                   .Names = c("Unique_ID", "Distance.km.", "Difference"),
                   class = "data.frame", row.names = c(NA, -10L))
My_data_table
Unique_ID Distance(km)  Difference    
1AA        1             NA          
1AA        2.05         1.05          
3AA        2             NA          
3AA        4             2          
5AA        2             NA          
5AA        4             2          
5AA        7             3   
6AA        8             NA
6AA        9             1
6AA        10            1

```

Here is the result i'm looking for 

```
      My_data_table
Unique_ID Distance(km)  Difference            
3AA        2             NA          
3AA        4             2          
5AA        2             NA          
5AA        4             2          
5AA        7             3   

```