If you are confident in programming likelihood functions, then NLMIXED is a strong possibility.
However, you can use PROC GLIMMIX with joint distributions for the variables in the panel (I assume there is more than one dependent variable). As an example, check out Example 41.5 Joint Modeling of Binary and Count Data in the SAS/STAT12.3 documentation for PROC GLIMMIX. Although the dependent variables here are from distinctly different distribution families, the method could be generalized.
Steve Denham